When we try to create a table which already exists which also adds
indexes, then the `if_not_exists` option passed to `create_table` is
not extended to indexes. So the migration results into an error if the
table and indexes already exist.
This change extends the `if_not_exists` support to `add_index` so that
if the migration tries to create a table which also has existing
indexes, error won't be raised.
Also as a side-effect individual `add_index` calls will also accept
`if_not_exists` option and respect it henceforth.
[Prathamesh Sonpatki, Justin George]
7ba08037f8