I didn't know that mysql doesn't support transactional ddl.
It means that we cannot create table, alter table and etc. in
transaction. At the moment migra supports only postgres client,
that can be use transaction for ddl.
I added a single transaction option for apply, upgrade, and
downgrade commands, which wraps all migrations into a single
transaction. This gives you the ability to safely roll up
migrations and, if some unforeseen situation occurs, roll them back.
Unfortunately if there is an error in syntax, mysql will not
rollback the migration and commits automatically :( I will
research this issue.
Closes#2
