Default urlpath/filepath configurable from a file in repo?

[consideRatio]

Wouldn't it be proper to have a setting to indicate what the default urlpath / filepath should be for repo2docker? It is currently a logic on the binderhub level, but I figure it makes sense to have support for this within repo2docker directly as well.

I just built a repo with jupyterlab etc isntalled with repo2docker (master) and entered the classic Jupyter UI, somewhat of a letdown.

[betatim]

Maybe a starting point is to have a --filepath and --urlpath argument to repo2docker that let you configure this. Most of this would be editing the URL printed by the notebook at the end. Unsure how hard it is to do that.

[manics]

One option is to make this part of the specification for describing a decoupled UI: #868