Extend `SimpleColor` to hold an alpha component

[tecosaur]

I'm not sure whether or not it's a good idea, but adding an alpha component to SimpleColor (i.e. making it rgba) seems interesting.

LaTeX doesn't (easily) support transparent colours, HTML absolutely does, and some terminals do in certain ways (e.g. being able to set a translucent background colour).

For now, this issue serves as a stub to collect thoughts.

[tecosaur]

Alternatively, we could add an alpha component to the Face struct, so it's possible to change it independently of the shade.