WIP/ENH: add tool to find changes to a single item (or multiple items)

[klauer]

This is the alternative implementation as noted in #45

• It looks very similar because it started off from https://github.com/pcdshub/all-deployed-iocs/blob/master/track_ioc_changes.py
• And then I apparently had the same idea to implement the contents of diff.py

This implementation is a little different, though:

• You can track changes for a single happi item
• Or you can track changes for all happi items on a per-commit basis
  • It can automatically determine (at least some) renames by looking at the git commit diff

[ZLLentz]

Maybe we can merge both of these? They don't seem to conflict with each other.

[klauer]

We can merge both and circle back if one ends up being a clear winner