ItemSuggest: Add custom filtering

[stefandesu]

One feature that is available in Cocoda, but not yet in the ItemSuggest component, is filtering. For example, when searching for concepts, we could add a type filter. When searching for vocabularies, there could be a lot of different filters (note that filtering and searching can't currently be combined: gbv/jskos-server#144).

Ideally, this would be implemented in a way that makes it easy to add custom filters.