Multiple styles

[yonaskolb]

This allows you to register multiple styles for the same strings. This is useful when grouping different styles ie (all these get shadows), and then setting other properties in other closures.

This pull request also allows you to clear styles, and fixes some swift warnings

[vadymmarkov]

@yonaskolb Looks great! Could you please add some tests?

[yonaskolb]

Added tests.
Technically this is a breaking change as someone who is relying on the fact that re-registering a style will overwrite it, will find different behaviour here

[vadymmarkov]

Thanks @yonaskolb. I'll release a new version soon.