Ext filter

Add ExtFilter, which implements filtering by file extension. Add dedicated needles for extension; that way, we can use nice, cacheable term filters.
Test passed. :-) That is correct, at least at a first level. Of course, I wouldn't be surprised if there were corner cases in ES's 'not' filter that would make that optimization invalid: for example, if there were not only a 'not' key but another on the same level that was important.