Improving link and dlink

`link` and `dlink` are good for simple uses, but lack certain features

1) They are not compatible with `Output` widgets, making debugging a lot harder, see https://github.com/jupyter-widgets/ipywidgets/issues/2306
2) They contain redundant code
3) They are built around the "lock" pattern, but in an ad-hoc way: there's no context manager. Such a thing would be quite useful for a user needing a slightly more sophisticated linking pattern, say, linking a 2-Tuple trait with 2 Tuple traits, etc.

Is there any interest adding some optional arguments to link and dlink, and creating a `Lock` context manager?

I would be happy to do a PR