Issue #32: filter possible values come from trek data, so that only useful sort values are displayed
Issue #32: doing some filter DRY fixes + adding usages/route/valleys/theme filter logic
Issue #32: moving filter logic to treksFilterService, to keep controller as clean as possible
Issue #32: removing filter duplicate values + alphabetical sorting
Merge pull request #34 from makinacorpus/dynamic_filters

Issue #32: Dynamic filters