Add support for 'dir' param in hooks. Fix bug with var parsing.

[brikis98]

Description

This PR extracts two improvements from https://github.com/gruntwork-io/boilerplate/pull/125:

1. Add support for a dir parameter in hooks for specifying the working directory.
2. Fix a bug with --var parsing where it wasn't handling an = in the value correctly.

Since https://github.com/gruntwork-io/boilerplate/pull/125 is a hackday and not likely to merge soon, I moved these improvements into a separate PR so we can get them merged ASAP. Also, in this separate PR, I added automated tests for these improvements.

TODOs

Read the Gruntwork contribution guidelines.

• [ ] Update the docs.
• [ ] Run the relevant tests successfully, including pre-commit checks.
• [ ] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [ ] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[brikis98]

Thanks for the review! Merging now.