Add custom grouping based on explicit filter groups

This is very similar to #135 (closed), but more verbose, which makes it more flexible/powerful.

The idea here would be that the user would write an explicit, top-level or filter, and each bracketed subfilter would be a group. For example:

(A or B) or (A and D) or C

would produce three groups:

1. A or B
2. A and D
3. C