RNThemeButton label shadows and 'CapInsets' key detection for images.

Apologies if the format of this pull request is incorrect, it's my first one!

I think you're work so far on this system is great, I've been thinking of implementing something similar for a while.

I've expanded the types of keys the system is capable of handling to include CGSize and UIEdgeInsets. - See the commit message.

Do you approve of the additional implied key for image CapInsets?

Many thanks.