Added Font Addon

Not sure if this fits with your idea of things, but I needed this internally regardless so I thought I'd submit a pull and see if you wanted it. I was tempted to make this a separate module, but the way cgo and package-types/type transformation works I couldn't figure out how to implement allegro.Color in so this is builtin to the main allegro package.

This implements most of the important methods from the font add-on. Namely:
- InitFont() for al_init_font_addon
- InitTTF() for al_init_ttf_addon
- LoadFont(filename, size, flags) for al_load_font
- Font.Destroy() for al_destroy_font
- Font.DrawText(text, color, x, y, flags) for al_draw_text
- Font.TextWidth(text) for al_get_text_width
- Font.TextDimensions(text) for al_get_text_dimensions

Hope this is useful!