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