Add support for multi-column indexes using `add_index`

[mikeeus]

I don't think this is important atm since we can use raw sql:

    execute "CREATE UNIQUE INDEX chapters_section_id_code_index ON sections USING btree (section_id, code);"

But I'd like to keep it here so I don't forget about implementing it when I have a chance. This might also be a good first issue for someone whose up for it.

[paulcsmith]

I love this idea! 👍

create_index :users, [:name, :email]