Context-specific completer options. Option refactoring.

New review
Owner: dougkearns
Hopefully the changeset hash will actually be linked this time. If
not, it's the tip of the testing branch, presumably r4161.
I've been meaning to do this for a while. It allows case matching,
sorting, and auto completion options to be assigned on a per context
basis, with fine grained control. It also adds builtin regex support
to options since it's used fairly extensively. There are definitely
other options that would benefit from the regexlist type, if I can
dig them up. The interface (and perhaps the doc wording) is the only
place I'm really ambivalent.

--HG--
branch : testing

common/content/bookmarks.js

@@ -598,6 +598,18 @@ const Bookmarks = Module("bookmarks", {
                     context.completions = [["", "Don't perform searches by default"]].concat(context.completions);
                 }
             });
+
+        options.add(["suggestengines"],
+             "Engine Alias which has a feature of suggest",
+             "stringlist", "google",
+             {
+                 completer: function completer(value) {
+                     let engines = services.get("browserSearch").getEngines({})
+                                           .filter(function (engine) engine.supportsResponseType("application/x-suggestions+json"));
+
+                     return engines.map(function (engine) [engine.alias, engine.description]);
+                 }
+             });
     },
     completion: function () {
         completion.bookmark = function bookmark(context, tags, extra) {