GH-359: Fix beforeload to work with POST requests - #367

### Platforms affected
iOS and Android


### What does this PR do?
Fixes the behaviour of `beforeload` to resolve the problem with POST requests outlined in apache#359.

The `beforeload` parameter has been changed from taking only a boolean (`yes` or not defined) to a discrete string with possible values of `get`, `post`, or `yes` which correspond to request types of GET, POST or GET&POST respectively. The `README.md` has been updated to reflect this.

Note that use of `beforeload` to intercept POST requests is currently not supported on Android or iOS, so if `beforeload=yes` is specified and a POST request is detected as the HTTP request method, `beforeload` behaviour will not be applied. If `beforeload=post` is specified, a `loaderror` event will be dispatched which states that POST requests are not yet supported.

#### Notes for Android

The `shouldOverrideUrlLoading()` override method has been updated to support the [new method interface added in API 24 / Android 7][1] which receives the `WebResourceRequest` instead of just the `String url`, enabling the HTTP method of the request to be determined. The [deprecated method interface][2] has also been preserved for API <=23, but in this case the HTTP method cannot be determined so is passed as null.

Also note that due to a [Chromium bug](https://bugs.chromium.org/p/chromium/issues/detail?id=155250),  `shouldOverrideUrlLoading()` is currently not called for POST requests. It's possible this may be resolved in a future Chromium version in the Android System Webview (given that this is now self-updating and independent of Android version since Android 5.0) - in prospective anticipation of this, code to handle POST requests has been added to `shouldOverrideUrlLoading()`.

However, it seems more likely that this won't be resolved any time soon given that [a Chromium dev said](https://bugs.chromium.org/p/chromium/issues/detail?id=155250#c39):

 > We're looking at implementing a better way to handle request interception in a future OS version. There's no way to just "fix" this, the API doesn't accommodate this usage at all. This will not be something you can use any time soon.

Therefore if we want to go ahead and use `beforeload` to intercept request types other than GET, it's likely we'll instead need to use the `shouldInterceptRequest()` method override. As with `shouldOverrideUrlLoading()`, there are a two variants: the [new method interface][3] added in API 21 / Android 5.0 which  which receives the `WebResourceRequest` object and the [deprecated one][4] which receives only `String url`. If we want to determine the HTTP request method, we'll need to use the new implementation. This has been empirically tested and *is* called for POST requests so would allow the possibility to intercept, delay, modify and send the POST request and its data via `beforeload`.
Both `shouldInterceptRequest()` method interfaces have been exposed in the Android implentation for potential future use but they currently do nothing other than return the unadulterated request object.

### What testing has been done on this change?
Manual testing of POST and GET requests on both platforms using a test app container:
https://github.com/dpa99c/cordova-plugin-inappbrowser-test

[1]: https://developer.android.com/reference/android/webkit/WebViewClient.html#shouldOverrideUrlLoading(android.webkit.WebView,%20android.webkit.WebResourceRequest)
[2]: https://developer.android.com/reference/android/webkit/WebViewClient.html#shouldOverrideUrlLoading(android.webkit.WebView,%20java.lang.String)
[3]: https://developer.android.com/reference/android/webkit/WebViewClient.html#shouldInterceptRequest(android.webkit.WebView,%20android.webkit.WebResourceRequest)
[4]: https://developer.android.com/reference/android/webkit/WebViewClient.html#shouldInterceptRequest(android.webkit.WebView,%20java.lang.String)