[pass] Rename command

[Matthieu Weber]

Hello,

I added a "rename" command (aliased as "mv") that allows to rename a .gpg
file. It uses "git mv" if available to rename the file, otherwise uses "mv".
It does not allow to move a file to a directory. E.g., if you have "foo.gpg"
and a directory "Dir", you must write "pass mv foo Dir/foo". It is not allowed
to write "pass mv foo Dir". It should otherwise be quite straightforward.