Allow use as a git merge tool

[ChristopherKing42]

Git has built in support for interfacing with external merge tools. We should add support for that:

https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration#External-Merge-and-Diff-Tools

[smashwilson]

There's already been a script created for this in #195, but it isn't exposed in any obvious way yet.