Add command to rollback migration #547

It would be nice to have a command that rolls back to a previous version. This should work like the git reset <hash> command.

Syntax-wise, we could use the git style (which is what Django uses), where we specify the file name to rollback to:

cot migration rollback m_0001

and support another variant of the git-style (which Laravel uses), where you specify the number of migrations to roll back.

cot migrations rollback --step 3