fixes #371

* Don't allow the scrollview to scroll the WebView body.

* Readme comment

* Remove duplicate protocol def

This PR fixes two crashes when enableWebKit is true (which was the default) on iOS 9.x. The first one when the WebView component was mounted (fixes issue #150) and the second one, when the component was unmounted (fixes issue #213).

The first problem happen when the RNCWKWebView.m was loaded:

The programming guide for WebKit (version 2006) ([pdf](http://mirror.informatimago.com/next/developer.apple.com/documentation/Cocoa/Conceptual/DisplayWebContent/WebKit_DisplayWebContent.pdf)) from Apple said, that it is (was) required to check if the 'new' WebKit Framework was available. This was required when the WebKit framework was only available on Mac OS X (10.2) when the Safari was installed. 😆

Because WebKit is (currently..) a fix part of iOS we didn't need this check this anymore to determinate if WebKit is available. I also see no other reference that this is required in iOS so I just remove this code. Without this code the WebView works also on iOS 9.x.

The second issue happen when the WKWebView was removed from the native view hierarchy. Its required (only on iOS 9) to remove the UIScrollView delegate before cleaning up the webview.

Its possible to try this PR with:

```
npm install --save "react-native-webview@jerolimov/react-native-webview#fix-ios9-crash"
# or
yarn add "react-native-webview@jerolimov/react-native-webview#fix-ios9-crash"
```

If you use CocoaPods, you should also update your Pods with

```
cd ios && pod update && cd ..
```

* In order for TLS Mutual Auth to work for webviews, the caller must present a credential. Expose a setter that can be called to set a credential.

fixes #304

fixes #29
fixes #272
fixes #221
fixes #105
fixes #66

BREAKING CHANGE: Communication from webview to react-native has been completely rewritten. React-native-webview will not use or override window.postMessage anymore. Reasons behind these changes can be found throughout so many issues that it made sense to go that way.

Instead of using window.postMessage(data, *), please now use window.ReactNativeWebView.postMessage(data).

Side note: if you wish to keep compatibility with the old version when you upgrade, you can use the injectedJavascript prop to do that:

const injectedJavascript = `(function() {
  window.postMessage = function(data) {
    window.ReactNativeWebView.postMessage(data);
  };
})()`;

Huge thanks to @jordansexton and @KoenLav!

Fixes #139

Added a new cacheEnabled prop to toggle Android & iOS webview caching behavior.

BREAKING CHANGE:  This change makes caching enabled by default when previously there was no caching behavior which may cause unexpected behaviour changes in your existing implementations.

Allows the webview to be opened with an ephemeral data storage.

* fix(WKWebview): [iOS] Add shared process pool so cookies and localStorage are shared across webviews (#68)

* Add optional shared process pool

BREAKING CHANGE: useSharedProcessPool prop is set to true by default. If you want the old behavior, please use useSharedProcessPool={false}

In the current code using `startInLoadingState` and `injectedJavaScript` will result in an infinite loading state if `injectedJavaScript` fails to evaluate for some reason. This adds a red box error explaining there was a failure to evaluate javascript. In my case this was do to the JS string not returning a valid type so I've added a that as a potential solution in the error message and added some documentation to the API Reference with some additional warnings.

To reproduce the existing behavior setup a webview with `startInLoadingState` and `injectedJavaScript` that returns an invalid type (in my case it returned a function). You should see an infinite loading state as `onLoadEnd` is never called.

Try the same with this branch and you'll get a nice red box error suggesting one potential solution to the problem.

![simulator screen shot - iphone 8 plus - 2018-11-28 at 15 09 25](https://user-images.githubusercontent.com/1944151/49193714-fccde100-f334-11e8-89dc-bf220e0adf.png)

* [ios] Add ‘allowsLinkPreview’ property to iOS WKWebView.

* Add divider in docs

* Typo: add hyphen -> _allowsLinkPreview

* fix webview always recreate

* fix webview always recreate

* code format

* Update RNCWKWebView.m

* @bugfix fix the message handle problem

* Add 'userAgent' property to the iOS WKWebView

* Update 'userAgent' reference docs.

* Fixes #81 
* Fixes #72

* add hideKeyboardAccessoryView option

* add hideKeyboardAccessoryView prop to reference

* Added the onLoadProgress property to the iOS wkwebview and Android webview