Add additional flags to openedx-atlas CLI tool to allow for specific commit and/or specific label selection

[Carlos-Muniz]

Reasoning:
Adding support for a specific commit/label may make it easier for smaller site administrators to use the exact versions of translation files they need. Specific directory, repo, and branches are already supported by openedx-atlas.

Contingencies:
This is a very low priority feature request as it was mentioned once, and can be added at any time.