Add filtering support

[carltongibson]

I'd like to add better support for filtering, similar to DRF. I have in mind Django Filter but a more generic filter backend interface would do too.

I've found first needing to implement get_context_data to pass the filter to the template. No problem here really.

But then in order to support pagination I have to implement get_paginator:

def get_paginator(self, queryset, page_size):
    return Paginator(self.filter, page_size)

filter here is a property, lazily instantiating the filter instance — I need it there because I still need the filter in the template.

Again, no real problem but it would be nice to just specify a filter_class, or filter_fields maybe... — in reality I'd guess most cases would merit a get_filter implementation, to get the customised filter instance — and have it Just Work™

The is kind of relevant to tomchristie/django-rest-framework#2180, where we need to keep the filter backends around in order to pass them to the template.

I'm happy to do all this if it's acceptable.

[carltongibson]

As per #39.

[ayushin]

I think adding an overridable filter_queryset() into get would add a lot of flexibility

[adamchainz]

Please don't comment on 6 year old issues that were closed with a sensible resolution.