Ability to run a command instead of opening bash

[kostajh]

I'd like to be able to use fresh in a pre-commit hook for my extension, so I could have something like fresh-node --entrypoint=grunt stylelint or fresh-node --entrypoint stylelint --fix **/*/.less

[Krinkle]

Sounds good to me.

A few quick notes:

• I added something like this unofficially for the test suite ("FRESH_INTERNAL_RUNCMD"). So I suggest we replace that with what we come up with here.
• Note that grunt or stylelint would not be available here. I think it would be good practice to maintain those commands in package.json instead and invoke npm run … instead. That keeps the code central, avoids hardcoding for node_modules in Fresh, and makes it easier for users to run those same things by other means if they don't want or can't use pre-commit hooks.
• Slight nitpick - "Entrypoint" might be too easily confused with the docker internals. Perhaps even simpler would be to consider all arguments after -- as the command to execute?

For example

fresh -- npm run lint
fresh -- npm run fix:less

[Krinkle]

Continue at https://phabricator.wikimedia.org/T253924.