Add drop_table to Clear::Migration::Helper

[watzon]

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There's already a create_table helper which is awesome, but it would be nice to have a drop_table as well for down operations. I could probably submit a PR.

[anykeyh]

I think there is a drop_table. Or I did it after you asked for it. You can still use dir.up { execute "..." } for operations not supported yet.

Anyway will close this issue once the v0.9 is released.

(oh yeah, reply after more than a year...)