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
At least you can do:

```js
defaultMetadata_(metadata = {}) {
  metadata.traceurOptions = metadata.traceurOptions || new Options();
```