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!

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.

* add test code for disable hardware acceleration

* add interface to disable android hardware acceleration

* Update index.d.ts

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

* add style doc in reference.md

* Update Reference.md

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

* Add 'userAgent' property to the iOS WKWebView

* Update 'userAgent' reference docs.

* add hideKeyboardAccessoryView option

* add hideKeyboardAccessoryView prop to reference

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