Add drop_enum(:my_enum_type, force: :cascade, if_exists: true)

[shaicoleman]

Hi, would be useful to support to support :if_not_exists for create_enum, e.g.:

create_enum(:my_enum, %w(opt1 opt2), :if_not_exists: true)

And force: :cascade and if_exists: true for drop_enum

Thanks

[bibendi]

Duplicates #23

[hosembafer]

@bibendi seems it's not duplicate of #23, because it's about create_enum and drop_enum, not about enum values. I opened PR for this issue, take a look #27