Allow referencing custom fonts by string or as a font resource reference

[G00fY2]

Currently the attributes for custom fonts are limited to font resource references only.

<item name="md_font_title">@font/your_font</item>
<item name="md_font_body">@font/your_font</item>
<item name="md_font_button">@font/your_font</item>

It is not possible to reference system fonts this way (e.g. the Roboto font family).

This PR is inspired by the TextView's android:fontFamily attribute and uses the same logic to resolve the typeface.

This PR will not interference with the current behavior, but it allows to additionally reference fonts by string:

<item name="md_font_title">sans-serif-black</item>
<item name="md_font_body">sans-serif-light</item>
<item name="md_font_button">@font/your_font</item>