Support create_enum in a change migration

[ronen]

Make create_enum reversable so that it can be used in a change.

Currently you must use separate up and down methods (with drop_enum in the down)

[Issue pulled over from SchemaPlus/schema_plus#177]

[kevinjalbert]

👍 to this feature. I think it should be supported, as my first use of this gem I got bit by the lack of this.

Might be worth putting a note of this in the README for the time being.

[ronen]

@kevinjalbert Unfortunately no time for me to work on this, but I'm happy to accept PRs. Even a PR to update the README!