Implement --global option for the 'setup' command

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Use a hash to map command names to subroutines

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Setup git-change.project option

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Rename git-change to git-gerrit

The name git-change is already taken:

    https://github.com/Nextdoor/git-change

I didn' want to change it because I think 'git-change' is much more
meaningful given that most of its subcommands can be thought of as
'methods' of a 'change' object.

The name 'git-gerrit' was my first choice and I'm going back to it
since, so far, I couldn't find it used anyware.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>