Allow inter-facet relationships

[dkavanagh]

The basic idea is that if one facet is selected, if filters another facet's options. Thinking about some simple additions to facet data structure to support this. For example, if facet A indicates facet B should be filtered by facet A's value, then facet B would have an additional value on each option to match one of the facet A values, so that facet B options would be reduced by selecting facet A.