add NO_COLOR env variable to avoid ansi escape codes

[kaffarell]

Sometimes ansi escape codes cannot be rendered (e.g. when executing
inside of vim with :!git who -l), so add a evironment flag which
doesn't print ansi escape codes. Also add the --color flag to override
the environment variable.

[sinclairtarget]

Hi kaffarell, I noticed your fork and created this issue: #39

Thanks for opening the PR and for pushing for this change. I think it's a good idea. My only suggestion is that we should follow the spec linked in the issue. NO_COLOR=1 should turn off color, and instead of having a --no-color flag we'd have a --color flag to enable color.

[kaffarell]

Thanks for looking at it, updated the PR.

[sinclairtarget]

Thank you! This is looking great.

One last thing: According to the spec, if NO_COLOR is set to the empty string then it looks like it's supposed to count as not being set. I observe that same behavior using bat:

$ NO_COLOR="" bat foo.go
<output is colored>

[kaffarell]

Done!

[sinclairtarget]

Nice! Thank you very much for your work on this.

I may wait to push out another release until there have been a few more changes. But I'm excited to get this out eventually!