Add support for remaining weights and styles

[The stable version of Raleway](https://github.com/impallari/Raleway/tree/master/fonts/v3.000%20Fontlab/TTF) contains 18 styles and weights (Thin, Extra Light, Light, Regular, Medium, SemiBold, Bold, Extra Bold & Black, each in roman and italic variations).

It'd be great to have all these variations supported by RT-Raleway.