Added support for the commands that I use heavily (gitk).
This pull request includes the support for gitk_command setting (already merged into the python3 branch) and usage of gitk for "this file" only (as opposed to gitk on the entire repository)
Added a gitk_command configuration value to specify the path for the …
mapped git_gitk command to work on the current file
added menu entries for gitk commands
@kemayo Let me know if there is any problem with this request, there are several open ones for gitk so I have updated mine to be the closest to current master