Migrations: change column
I think I will pass on this, but thank you.
 What wrong?
 Well, I think `ALTER TABLE / ALTER COLUMN` is just complicated enough that I'm not sure this implementation will suffice. Additionally, the MySQL syntax is quite a bit different IIRC, and is not added as part of this PR.
 That the reason why I didn't include the functionality in docs and chosen such word as `beta`. But the code (uncompleted of course) already helped me with postgres and sqlite.

But it up to you, coz. Thank you for the project!
 Thanks for the PR and for taking the time to submit the code. If I should decide to add this functionality in the future I will definitely use this PR as the starting place.

I've used the functionality in my project with Postgres and it works for me. So I thought it could be useful for other people. The request provides beta support for `change_column` functionality in migrator. I've tested this with Postgres and SQLite.
