Webview to be spun out of react-native

[jvliwanag]

RN webview now lives at:
https://github.com/react-native-community/react-native-webview

Tried running react native 0.58 and found two issues. Currently purescript-reactnative by virtue of importing webview directly from react native emits a yellow box warning. (The other issue should be handled by #27)

Not sure how to go with this though. Two options:

1 - Remove Webview bindings from this project. Let's maybe create a separate purescript-reactnative-webview
2 - Add a dependency to the js react-native-community/react-native-webview.

I think option 2 would mean users of this library would always need to install that js dep though even if they don't use the webview. So I'm leaning towards option 1.