Add OpenSearch suggestion support.

Add autocomplete support when searching Danbooru from the Chrome address
bar. If you type "danb<tab>" in the address bar then search for a tag,
then autocomplete results from Danbooru will appear as search
suggestions in Chrome.

Note that the "Autocomplete searches and URLs" Chrome setting must be
enabled for this to work.

Ref:

* http://dev.chromium.org/tab-to-search
* https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Supporting_search_suggestions_in_search_plugins
* https://github.com/dewitt/opensearch/blob/master/mediawiki/Specifications/OpenSearch/Extensions/Suggestions/1.1/Draft%201.wiki

app/controllers/autocomplete_controller.rb

@@ -0,0 +1,16 @@
+class AutocompleteController < ApplicationController
+  respond_to :xml, :json
+
+  def index
+    @tags = Tag.names_matches_with_aliases(params[:query], params.fetch(:limit, 10).to_i)
+
+    if request.variant.opensearch?
+      expires_in 1.hour
+      results = [params[:query], @tags.map(&:pretty_name)]
+      respond_with(results)
+    else
+      # XXX
+      respond_with(@tags.map(&:attributes))
+    end
+  end
+end