Add an optional base directory

[AlbertoPdRF]

This PR adds an optional base_directory from which to execute all commands. This is useful for deploying apps that live in monorepos.

To test that this works, I:

1. Forked this repository
2. Merged my changes into the master branch of my fork
3. Created a release including the changes
4. Used the action as part of a workflow on a test repository

This PR also includes some formatting changes made by Prettier, although I can get rid of those if necessary!

[perelin]

I could really use this right now. working-directory does not seem to work for building in a subdirectory.

[ewels]

Came here looking for this as well..

working-directory does not seem to work for building in a subdirectory.

I tried this too, but it's only for GitHub Actions run steps, so doesn't work here as we're with an action uses. In this case we need it to be built into the action that is being used.