Until now, buttons would use the same font family used by the title,
this commit introduces navBarButtonFontFamily property which allows to set
a different font family for title and buttons.

* Support subtitle FontFamily on size on Android

Font family can be either a .ttf/.otf asset or one of the default font families:

* sans-serif (regular)
* sans-serif-light
* sans-serif-condensed
* sans-serif-thin
* sans-serif-medium

* update docs

* add disabledButtonColor in order to set navigation button disabled color

* add disabledButtonColor to the docs

Ability to set a flat border under the TopBar. `topBarElevationShadowEnabled` should be set to true if using this feature.

[iOS] Added ability to set static(root) background image

added a missing comma in the 'style object format' so to help users who copy/paste this code block

Some navBar button properties were marked as common by mistake.

Separated iOS and Android only properties.

* Add ability to set attributes of navigation bar buttons separately.

* Added ability to set button style when calling `setButtons` method.

* Add documentation.

Reading the docs I couldn't see a way to simply change the color of the status bar on Android. Only via https://github.com/wix/react-native-navigation/issues/571#issuecomment-267255350 I saw there was a prop to do this.

(Assuming this is Android only since iOS only allows for light or dark).

* Initial commit

* Initial commit

* Clean-up title