A11y: support NO_COLOR environment variable in output

[MaddyGuthridge]

Resolves #302

This is a quick fix, rather than a major change to the system Morgan uses for styled console output. In an ideal world, using NO_COLOR would disable all styling of output, which would remove the presence of control characters in things like CI logs. Currently, this PR just sets the color to 0 if NO_COLOR is set in process.env, which is acceptable for accessibility.

As a side note: I'm not used to writing code for such old versions of Node, so apologies if I made any mistakes that break that compatibility. I tried testing using Node 0.8.28, but was unable to get npm i to work correctly. Let me know if I broke anything and I'll do my best to fix it up :)

• Implementation
• Test suite passes with 100% coverage
• Linting passes with no errors or warnings

I accept the certificate of origin, and am happy for my contribution to use the project's MIT software license.

[krzysdz]

I believe that NO_COLOR=1 should entirely remove the colour-related ANSI escape codes instead of emitting the reset sequence.

[MaddyGuthridge]

Makes sense to me, although that would require a bit of a bigger refactor. I am pretty flat-out right now so probably won't have time to sort that out for a few weeks at least. I'll keep it on my todo list though.