Add Plot.project("color") as an alias for "fill" or "stroke"

(it chooses intelligently).

This abstracts away the messy particulars of SVG, but is a little magical.

I'm not in a huge rush to get this one in.
Since the variable itself never changes, I'm ok with static... except that linePlot uses a different value...  How do we account for this case?

What do you mean by look it up on the plot constructor?