Add new command to diff only current file

 - Change `git-difftool:open` command to `git-difftool:diff-project`
 - Keymapping for `git-difftool:diff-project` changes to ⌥⌃⇧D
 - Introduce `git-difftool:diff-file` command
 - Keymapping for `git-difftool:diff-file` is ⌥⌃D

Closes #3