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

test/test_bazel.sh

@@ -27,5 +27,8 @@ bazel build //hello-world:hello-world-wasm-simd
 cd test_external
 bazel build //:hello-world-wasm
 bazel build //:hello-embind-wasm --compilation_mode dbg # debug
+
 # Test use of the closure compiler
 bazel build //:hello-embind-wasm --compilation_mode opt # release
+# This function should not be minified if the externs file is loaded correctly.
+grep "customJSFunctionToTestClosure" bazel-bin/hello-embind-wasm/hello-embind.js