This PR adds support for `if_exists` on `remove_column` and
`if_not_exists` on `add_column` to support silently ignoring migrations
if the remove tries to remove a non-existent column or an add tries to
add an already existing column.

We (GitHub) have custom monkey-patched support for these features and
would like to upstream this behavior.

This matches the same behavior that is supported for `create_table` and
`drop_table`. The behavior for sqlite is different from mysql/postgres
and sqlite for remove column and that is reflected in the tests.