feat: allow multiple directories

[SethFalco]

Currently, we're running the fpb-lint command 4 times to test each directory.

This change makes the command take a list of directories instead of just 1.
After this, we can update free-programming-books from having 4 separate run steps to just:

- - run: fpb-lint ./books/
- - run: fpb-lint ./casts/
- - run: fpb-lint ./courses/
- - run: fpb-lint ./more/
+ - run: fpb-lint ./books/ ./casts/ ./courses/ ./more/

The motivation is to collate the linting to a single process, output, and exit code.
This will make it simpler later when/if we make more use of the output, like suggesting fixes and automatic suggestions on GitHub.

By merging the 4 runs into 1, this also incidentally addresses an issue we have with GitHub Actions where the build fails early. If warnings were displayed in ./books/, it would fail and stop linting further files. This means there could be more warnings in ./casts/ but the contributor wouldn't know until after fixing the warnings from ./books/.

---

I added Commander as a dependency for 2 reasons:

• CLI parsing is a pain, this is simpler than accounting for spaces, quotes, escapes (\), etc.
• I'm hoping we can add more commands or arguments later. Namely, an argument to automatically apply fixes which can be used to automate suggestions in reviews.

[vhf]

Merged and release 6f39fdb