Warning about migrations altered after being run

[wbyoung]

This could be done by hashing migrations when they are run so we're able to warn the user about migrations that have been altered after they've been applied. Store the hash in the migration table & any time migrations are run, check that all previously run migrations still match their current values when read from the disk. This could be extended to ensure that when rolling back we prompt the user that this could potentially break and/or not reverse the migration as intended.

This would help for individuals new to migration systems (especially in teams) and seems like a nice thing to have.

[wbyoung]

It would probably be worthwhile to store a hash of the migration with all whitespace removed so that whitespace changes in the migration would be allowed.