Understand (and discard) SGR subparameters

[jfly]

This completes https://github.com/selectel/pyte/issues/178. pyte now understands :-delimited subparameters, but doesn't actually do anything with them (which is better than the previous behavior, where they'd get spewed to the screen buffer.

I've split the work here into 2 separate commits, each with some explanation in the commit message, I recommend reading them independently.

I've also laid the groundwork for actually using these subparameters, which I'm tracking in https://github.com/selectel/pyte/issues/179

[jfly]

@superbobry, do you have any time to take a look at this?