Add support for options in environment variables

[aidansteele]

This uses yargs' support for providing option values via the command line: https://github.com/yargs/yargs/blob/main/docs/api.md#envprefix

My use case is that in CI scenarios I want raw output, but locally I want prefixes. I could duplicate all the scripts in my package.json: one with concurrently -r, one without - but I feel that is suboptimal and could lead to mistakes when devs forget to keep them in sync.

With this change, my CI script can now look like:

set -eux
export CONCURRENTLY_RAW=true 

npm run check
npm run validate
npm run build-all
# etc..

[coveralls]

Coverage remained the same at 99.807% when pulling 4966bdc on aidansteele:patch-1 into c04740a on open-cli-tools:master.

[gustavohenke]

oooh this is an awesome feature!! Great idea!

Do you think that we could have some docs and possibly some tests too?

[aidansteele]

Sure thing, I'll try add those today :)

[gustavohenke]

I've added the docs and skipped the tests for now, there's too much repetition in the binary spec file 😐