Add Git source plugin

[rshipp]

Add a Git source plugin, for pulling down repositories and checking the new files for YARA rules.

• [x] Write subprocess-git functions for the git commands we'll need
• [x] Use the hash of HEAD for the saved_state
• [x] Get new files with diff --name-only -- $saved_state
• [x] Any new files that match .{yar,yara,rule,rules}, run through the YARA regex, and extract individual rules