added form for filtering

the idea here is to have (eventually) a set of pre-defined filters the
user can choose from and combine at will

however, this illustrates how tricky it is to represent such states (the
current selection of filters as well as the addition of new ones) in the
URI and constructing a suitable form to generate that URI - when that's
done, creating a corresponding JavaScript component to add filters
dynamically shouldn't be too hard