Support for ColorScheme #279

Mentioned way back in #5 (comment), it would be great if ColorScheme could be migrated from cli/cli's IOStreams. I was going to start a fork since I find myself repeating much of this often, but realize with x/color you might have something in mind. If there's a design in place, I'd love to take that into account and make sure it's threaded through all the places here currently like tableprinter and, at least to some degree, x/markdown — though, seems that will probably need more extensible support or at least a more general RGB color value, which is hard to make theme-aware generically.