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
I'm not sure how to express "default to `new Options()` for metadata.traceurOptions whether or not metadata has other properties.