Add configuration option search_tokenizer_separator

The default is for hyphens to separate tokens in search terms: `gem-based` is equivalent to `gem based`.

This adds `search_tokenizer_separator` as a site configuation option, to support search for hyphenated words.

assets/js/just-the-docs.js

@@ -53,7 +53,7 @@ function initSearch() {
       // Success!
       var data = JSON.parse(request.responseText);
 
-      lunr.tokenizer.separator = /[\s\-/]+/
+      lunr.tokenizer.separator = {{ site.search_tokenizer_separator }}
       var index = lunr(function () {
         this.ref('id');
         this.field('title', { boost: 200 });