Fix `String.prototype` polyfills

Ref. #562.
Rename RequireJs and NodeJs transformers and options

The options are now cjs and amd.

The transformers are now AmdTransformer and CjsModuleTransformer.

Fixes #547
Replace Project with Loader in repl.html

Adds InterceptOutputLoaderHooks module for grabbing transcoded source and map.
Changes Loader.evalAsync() to Loader.module(source, options) closer to spec.
Adds name to Loader.eval() calls, as proposed in https://github.com/jorendorff/js-loaders/issues/82.

Review URL: https://github.com/google/traceur-compiler/pull/564

Closes #564.