[WIP] Added convenient way to filter search results

This adds filters to package_search and utilizes it via the the regular web search.

filters is a dictionary of lists
{'topic': ['http://dbpedia.org/Obama', 'http://purl.org/ontology/bibo/']}

Each filter will be included in the SOLR fq parameter automatically when a search facet for it is defined.

Extensions can utilize filters to modify the search queries and results, too.

Old behavior is preserved.