make sure git diff doesn't print out with colors

[thatkookooguy]

resolves #30

[rtfpessoa]

That case only works for the default diff2html, if you define something after -- it will not pass the option.
You need to check the first if case. Might be nice to check if the args passed already include it and if not add it.

[thatkookooguy]

tested locally (by installing with npm install -g https://github.com/Thatkookooguy/diff2html-cli.git#patch-1).
But you know this code better than me. So you might want to check edge cases that I didn't think about :-)

[rtfpessoa]

Sorry for the delay. LGTM 👍

[rtfpessoa]

Released as 2.5.4