add_downs, plugin to help writing 'down' methods

Hi --

I've written a little plugin, partly just for fun (though there's no
reason it couldn't be useful), called add_downs, which lets you do

   rake db:migrate:add_downs

and end up with your migrations with their down methods written

The way it works is that it overrides create_table, add_column, etc.,
so that they print out their opposites, and then it executes your up
methods. It doesn't connect to the database, so it can't reverse
certain operations involving column type.

Overall it's mostly for simple-ish cases, but could be handy. You can
find it at: