Support for index definition in version table

[gauravrajfm]

I want to run custom queries on version table using the columns defined in the base table. By default version table doesn't have indexes defined on those columns. If I add custom migration commands in alembic script, it conflicts with the version table definition. It will be better if we can define a metadata in the custom table which indicates Continuum that index has to be defined on the version table as well.