Add ability to get sound counts from the results of a diff

[Kuoxsr]

When adding large quantities of sound files to a pack, I'd like to be able to do a git diff of the last commit before the previous release and the current state, and have spcheck verify that text instead of the entire current file. This would allow me to get a nice, formatted sound count of only the changes since the last release. Doing this manually is tedious and error-prone.

This might be better accomplished using grep with awk or something to build a .json file containing only the differences.