Introduce onCanGoBackChange & onCanGoForwardChange props

[mkhazov]

PR to origin repo: CRAlpha#210

WKWebview does not trigger onProgress and onNavigationStateChange when navigation occurs in SPA.
The issue is solved by adding key-value observers for canGoBack and canGoForward values and passing them to corresponding functions passed as props from React.

<WKWebview
  onCanGoBackChange={ (canGoBack: boolean) => {} }
  onCanGoForwardChange={ (canGoForward: boolean) => {} }
/>

[Enet]

Please don't forget to create PR and add comment on stack overflow.