Improve StateLabel props

[broccolini]

The StateLabel props are inconsistent and have several props that apply the same styles. They should just have scheme options for the color, and size props. Invalid is odd when the other props are color names. state as a prop name may cause confuse.

I think we should discuss how to handle the octicons and whether there should be one prop that handles both color an octicon or not, and/or whether they should just use the octicon component in the state label etc.