Add simplistic command structure

Now instead of using `gitto --register' or `gitto -r' one would use
`gitto add'. Here is the list of what was and what is:

--version, -v      => version
--help, -h         => help
--register, -r     => add
--remove, -R       => remove
--repositories, -l => list locations
--purge, -p        => purge
--check, -c        => check
--config, -C       => config
--global-config    => config global
--update-config    => config update

Running gitto without arguments keeps the same functionality, though
it can also be called as `gitto list'.