Support a url field in plugin metadata

spec/components/plugin/metadata_spec.rb

@@ -9,6 +9,7 @@ describe Plugin::Metadata do
 # about: about: my plugin
 # version: 0.1
 # authors: Frank Zappa
+# url: http://discourse.org
 
 some_ruby
 TEXT
@@ -17,6 +18,7 @@ TEXT
       expect(metadata.about).to eq("about: my plugin")
       expect(metadata.version).to eq("0.1")
       expect(metadata.authors).to eq("Frank Zappa")
+      expect(metadata.url).to eq("http://discourse.org")
     end
   end