Add detection for opa (http://www.opalang.org/).

test/fixtures/hello_syntax1.opa

@@ -0,0 +1,5 @@
+server = Server.one_page_bundle("Hello World",
+       [],
+       [],
+       _ -> <h1>Hello, world</h1>)
+

test/fixtures/hello_syntax2.opa

@@ -0,0 +1,7 @@
+Server.start(
+   Server.http,
+   {
+     page: function() { <h1>Hello, world</h1> },
+     title: "Hello, world"
+   }
+)