Add platform specific handling of font files

[noinfo]

Makes webfonts work in iOS and Android besides having to use different names in 'CSS' style.

[sibelius]

Can we make the user pass this as a prop instead? Having the default as the old value?

[noinfo]

Of course it’s technically possible but isn’t it rather clunky and unwieldy?

I mean the user would have to manually use a Platform.select whenever he uses an icon. That would make the whole thing go from easy and simple to use to quite cumbersome - it’s not even like the user could use it as is if he wants to support Android with his app he’ll have to set the prop on every single use - and with a platform.select and both possible values. Furthermore the user has to be taught which values he has to input. At that point it’s easier to just roll your own icon component...

[sibelius]

check if this helps https://github.com/entria/react-native-fontawesome/pull/61