Add support for optional marked content.

Add a new method to the API to get the optional content configuration. Add
a new render task param that accepts the above configuration.
For now, the optional content is not controllable by the user in
the viewer, but renders with the default configuration in the PDF.

All of the test files added exhibit different uses of optional content.

Fixes #269.

Fix test to work with optional content.

- Change the stopAtErrors test to ensure the operator list has something,
  instead of asserting the exact number of operators.

src/core/worker.js

@@ -481,6 +481,10 @@ class WorkerMessageHandler {
       return pdfManager.ensureCatalog("documentOutline");
     });
 
+    handler.on("GetOptionalContentConfig", function (data) {
+      return pdfManager.ensureCatalog("optionalContentConfig");
+    });
+
     handler.on("GetPermissions", function (data) {
       return pdfManager.ensureCatalog("permissions");
     });