Better primitives for custom filter behaviors

[gordonwoodhull]

We have single select (as the common custom behavior of onclick filter null then filter x), toggle select, and a couple range selects. But what if you want something in between?

It would be great to have an API for manipulating the filter set and then committing it.

I guess automatic commit (if that's what we want) would imply that it's a callback or event.

The obvious primitives are: add, remove, toggle, union, intersect. We already have these but they are not exposed conveniently or safely.

This complements #557. #557 makes the underlying primitives pluggable, but you would still have to fetch them and call them e.g chart.addFilterHandler()('b') - and then remember to redrawGroup. Pretty ugly.

Instead, call something like setFilters, passing a function which itself gets passed add, remove, and toggle.

The current toggle filter behavior (filter) can be written in terms of this.

[gordonwoodhull]

http://stackoverflow.com/questions/25996357/apply-multiple-filters-except-others-in-crossfilter-dc-js