Support "drop value from a type" operation

[antedeguemon]

Description

It would be good if ecto_enum_migration supported the drop value from type - the inverse operation of add value to type.

Postgres does not support this operation, so ecto_enum_migration would have to do it by dropping the type and re-creating it.

I don't know if this is possible because of dependent columns, but would be a nice feature, especially for migration rollbacks of add_value_to_type.