Use multiple search indices

Hi,

Thanx a lot for your bundle!

I'm using search indices for two fundamentally different entities. So I extended this bundle to be able to use multiple search indices with their own configuration.

This pull-request contains some code that is only to avoid BC breaks, but could eventually be removed, as updating a project to work with the new code would be very simple.

Will you consider merging this PR into your bundle? => if so, I'll add some unit-tests for the new configuration.

Best regards,
Dominik