Added color markup.

Continuing from discussion https://github.com/libgdx/libgdx/issues/908

Markup syntax:
- **[name]** Sets the color by name. Names are currently hard-coded into a private static `ObjectMap<String,Color>` containing the same colors defined as constants in the Color class.
- **[#xxxxxxxx]** Sets the color specified by the hex value xxxxxxxx in the form  RRGGBBAA.
- **[]** Sets the color to the previous color (kind of optional end tag)
- **[[** Escapes the left bracket.

Color names cannot be empty, cannot start with `#` and cannot contain `]`. It can contain unescaped `[` though.

Markup is disabled by defaut. Use BitmapFont.setMarkupEnabled() to turn it on/off.

Support for user defined color names not implemented yet. Should they be stored in a static or member map?