Make ansi colors compile time feature

[matklad]

I've noticed that we always configure tracing tree with with_ansi(false) in rust-analyzer, so I think we can shave a couple of dependencies off here!

[davidbarsky]

That seems like a candidate for a cargo feature! I'll try to get to this tomorrow.

[davidbarsky]

Bit of a late update on this: I gave this a spin, but separating all the ansi-formatting functionality to exist behind a feature flag is kinda annoying, and at the time, I didn't really have a good idea how to handle it. I do now (based off a pattern I saw Eliza do in tracing-subscriber), but I think it won't make it into today's release. I'm sorry about that!