Add metadata property to Loader function calls.

Pass options on metadata.traceurOptions.
This aligns us with spec changes for metadata and with es6-module-loader.

Refactor InternalLoader.load/load_ to simplify calls.
Clean up some error-usability issues from MultipleErrors change.

TBR=@arv
done.  I like this better than the `undefined` suggestion because it expresses non-default behavior.