Add zsh command completion function

Add completion for gitto (sub)commands and arguments for users of the Z shell.

Here is a little something to give back to the gitto project as a thank you for being such a cool tool. I did not bother to change the Makefiles to install the function in an appropiate location, e.g.: `/usr/share/zsh/site-functions`. Since I think that is something you'd prefer to add yourself and I am also not entirely sure if you like the current location `zsh/_gitto` in the project path.

Of course, let me know if there's something you'd like for me to change.