Allow TRUNCATE for MySQL

[ping-localhost]

Resolves issue #864 which originates from #656.

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I agree that proper isolation is a must, but my database forces safe updates, which the current DELETE-query does not adhere to. To resolve that, I've added a simple switch to choose between DELETE and TRUNCATE.

Not sure if this is the best solution (we could also wrap the DELETE with SET SQL_SAFE_UPDATES), so feel free to edit/comment/etc.

[coveralls]

coverage: 59.281% (+0.06%) from 59.226%
when pulling 55c859d on ping-localhost:added-safe-update-to-mysql-driver
into a5dc5d1 on golang-migrate:master.

[ping-localhost]

CI lint failure is not due to my changes, but because master itself is not compliant.

[emeka]

Could this be solved by adding a LIMIT clause to the delete instead of moving back to truncate which is not transactional?

[ping-localhost]

Closing this in favor of #1070