Feature/branch lookup by commit

[madeline-scyphers]

Summary

• Add method commits_in_tags, that analyzes all commits that make it into a tag.
  • """Analyze each tag, and trace backwards from the tag to all commits that make
    up that tag. This method looks at the commit for the tag, and then works
    backwards to that commits parents, and so on and so, until it hits another
    tag, is out of the time range, or hits the root commit. It returns a DataFrame
    with the branches"""
• Add ability for ProjectDirectory to take a list of Repository instances directly.
• Add repository parameter labels_to_add, which are extra labels to add to outputted DataFrames in addition to repo names. Which can be useful especially for ProjectDirectory, where you have several repos that fall under similar categories, so you can group them together.