Add Keyboard api facade

[zakpatterson]

Facades {Keyboard} from 'react-native'

With this patch, one can write something like the following, and get keyboard dismissing behavior when the user clicks in the background.

  def KeyboardDismissingView(mods: TagMod[Nothing]*) = TouchableHighlight(
    onPress = () => Keyboard.dismiss(),  // Keyboard.dismiss() added by this PR.
    style = literal(
      flex = 1,
    ),
  )(mods: _*)

[shadaj]

Thanks for the PR @zakpatterson, this looks really great! Just one small change to the type definitions, but otherwise looks good to go!

Could you also add a line to the changelog for these changes?

[shadaj]

Looks great! Thanks again for the PR!

[zakpatterson]

Thank you for this library and being open, helpful, and responsive to PRs. I hope to work on this a bunch. Thank you thank you.