Add an ES6Transpile middleware.

This uses the mtime of the files in `packages_es6/` to determine if a re-transpile is required. If no files exist in `packages_es6/` or the `bin/transpile-packages.js` doesn't exist this is a no-op.

This will be used by an upcoming PR to Ember allowing the usage of ES6 modules within the `packages_es6` directory.