[WIP] Custom fonts support in TextStyle fixes #6

[kkalisz]

Hi,

this is draft of support for custom fonts in TextStyle.
My main concern now is about how to ( or even should we ) support custom font with and ** fontStyle** at the same time.
Basically we can include different types of fonts with only one text style, or with multiple font styles. I think that currently there is no even easy way to do it on any of the platforms. We can use typeface on android and symbolicTraits on iOS and add some fallbacks to default font. But personally it think that (at least for now) we should skip fontStyle when custom font is set.
Main reasons for that:

• mocko-resources requires adding font files with single fontstyle
• combining both properties will add additional complexity
• in most cases developers will add/set custom font with given font style directly ( not by combining both properties)

What do you think ?

I will continue my work, when there will be decision which path should we follow.