Log visualize phases

[Stefan-Hanke]

This PR implements a part of #227 - namely, visualize phases and changes thereof.
I'm no designer, but with this, the current phase and changes thereof are clearly visible.

Checklist

• Use a separate branch in your local repo (not master).
• Test coverage is 100% (or you have a story for why it's ok).

Note

Nicolo will be away for 2-3 weeks starting July 16th 2018, so expect some delays in getting a response.

[Stefan-Hanke]

oh my - I relied too much on the prepush hook to catch errors. First prop type validation, and now uncovered lines. 👎

[coveralls]

Coverage remained the same at 100.0% when pulling 94edef8 on Stefan-Hanke:log-visualize-phases into 52be2c3 on google:master.

[nicolodavis]

Hmm, the colors get a bit jarring for longer logs.

I have an alternative proposal in https://github.com/google/boardgame.io/tree/log-phases that looks like the following (turns on the left and phases on the right; it also has the nice property of conveying to the user the fact that turns and phases are orthogonal concepts):

What do you think?

[Stefan-Hanke]

Looks ok to me.

[nicolodavis]

Ok, I'll get that branch in and close this then.