implement the auto_discovery_link_tag liquid filter (feature #206)

lib/locomotive/liquid/filters/html.rb

@@ -3,6 +3,20 @@ module Locomotive
     module Filters
       module Html
 
+        # Returns a link tag that browsers and news readers can use to auto-detect an RSS or ATOM feed.
+        # input: url of the feed
+        # example:
+        #   {{ '/foo/bar' | auto_discovery_link_tag: 'rel:alternate', 'type:atom', 'title:A title' }}
+        def auto_discovery_link_tag(input, *args)
+          options = args_to_options(args)
+
+          rel   = options[:rel] || 'alternate'
+          type  = options[:type] || Mime::Type.lookup_by_extension('rss').to_s
+          title = options[:title] || 'RSS'
+
+          %{<link rel="#{rel}" type="#{type}" title="#{title}" href="#{input}" />}
+        end
+
         # Write the url to a stylesheet resource
         # input: name of the css file
         def stylesheet_url(input)

spec/lib/locomotive/liquid/filters/html_spec.rb

@@ -8,6 +8,16 @@ describe Locomotive::Liquid::Filters::Html do
     @context = build_context
   end
 
+  it 'writes the tag to display a rss/atom feed' do
+    auto_discovery_link_tag('/foo/bar').should == %(
+      <link rel="alternate" type="application/rss+xml" title="RSS" href="/foo/bar" />
+    ).strip
+
+    auto_discovery_link_tag('/foo/bar', 'rel:alternate2', 'type:atom', 'title:Hello world').should == %(
+      <link rel="alternate2" type="atom" title="Hello world" href="/foo/bar" />
+    ).strip
+  end
+
   it 'should return a url for a stylesheet file' do
     result = "/sites/000000000000000000000042/theme/stylesheets/main.css"
     stylesheet_url('main.css').should == result