Support custom comparison command

Issue #26 new
erikprice
created an issue

It would be nice to be able to customize the command used to compare files, rather than hardcoding FileMerge.

I have implemented a basic solution (which I am using to integrate with Changes.app for viewing diffs). Please feel free to pull the changes from changeset https://bitbucket.org/erikprice/murky/changeset/14a8f41f09dd/ if they are acceptable (if not, let me know if there's something I can fix).