Make it possible to add local aliases to mrgit

[pjasiun]

It would be great to be able to define custom aliases in mrgit, similar to git aliases. For instance, I use:

git master

What is define this way:

master = !sh -c 'rem=$(git config branch.master.remote) && git fetch --all -p && git checkout master && git reset --hard $rem/master && git submodule update'

It would be great to be able to do:

mrgit master

Which will do:

mrgit exec "git master"

I think that we can start from simple aliases, which assign commands with mrgit exec executions.