Add support for prepending now cli args

[shawnbot]

Let's test this on Primer CSS! 🤞

[shawnbot]

Some notes before I leave for the day:

1. We tested now docs in primer/primer and it worked (I think it was this deployment).
2. This should be running now docs in [WIP] Add Actions for linting, testing, and deploying 🎉 css#665 because the action has args = "docs", but this failure's logs suggest that it's just running now in the repo root.

[shawnbot]

Welp, this works in theory, but I think it's blocked on Actions not passing args properly to the entrypoint. I'm going to add a workaround to the README.