Update form.py
Add query_label to give each filter a unique label
Parse optional named filters from filter url

Old format fltN_M still works, but the new format flt_name_operation coexists side-by-side.
Remove unwanted change from previous branch
Factor _get_extra_args into _get_listing_args and overrideable get_filter_args
Don't enumerate filters if there aren't any.
Don't try to parse filter strings that don't contain any separators
Merge pull request #1 from bryhoyt/named-url-filters

Named url filters