Extend text filters

[yvolk]

In andstatus#1 @hosheinz suggested concrete solution and syntax to extend current keywords filter.

I agree that we need to extend filtering. We did this three-four years ago last time (see e.g. #223 )

The main stopper here is making the filtering easy for Users...
So I would start not from the code (We can code whatever we want!) but from discussion of the

1. List of features
2. Corresponding UI

So the new issue created for such discussion.