Add kwargs support to DropIndexOp autogenerate render

Add kwargs support to DropIndexOp autogenerate render

Fixed issue where dialect-specific keyword arguments within the
:class:`.DropIndex` operation directive would not render in the
autogenerated Python code. As support was improved for adding dialect
specific arguments to directives as part of :ticket:`803`, in particular
arguments such as "postgresql_concurrently" which apply to the actual
create/drop of the index, support was needed for these to render even in a
drop index operation. Pull request courtesy Jet Zhou.

Fixes: #849
Closes: #852
Pull-request: https://github.com/sqlalchemy/alembic/pull/852
Pull-request-sha: 6392a287179ed746f709ba1e0e07ccab3ea8e4c6

Change-Id: I9b602178c32d6c6a41c0dbe0969a19bd4fa329bd