Track undo buffer changes

[alanhamlett]

@secoif says, "idea: track changes to a file's undo buffer (at least in vim) for more accurate reporting."

[timoxley]

AFAIK, currently time only gets logged when you save but handily, vim can save a record of each edit in a "persistent undo" file. Perhaps wakatime could periodically check the persistent undo directory for modifications to files that have yet to be saved.