Add closure example for wasm_cc_binary (#941)

* Stub out addition of closure_externs_file option in wasm_cc_binary

* WIP

* args are passed, externs cannot be found

* try location

* update tests

bazel/test_external/hello-embind-interface.js

@@ -0,0 +1,3 @@
+Module.customJSFunctionToTestClosure = function(firstParam, secondParam) {
+	console.log("This function adds two numbers to get", firstParam + secondParam);
+}