Add ability to specify notebook server instead of Binder server

[SamLau95]

Right now, the JS API only allows specifying a Binder URL. To start a widget, the JS does three things:

1. Request a notebook server from Binder.
2. Request a Python kernel from the notebook server.
3. Connect the widgets to the Python kernel.

It should also be possible to specify the URL of a running notebook server as well. This will skip step 1), which happens to also be the most time-consuming step by far for initial widget render — step 1) takes something on the order of 10 seconds; steps 2) and 3) take around 100ms.

The author will have to set up a publicly facing notebook server themself, however, since binder.org requires users to go through step 1) before we can do step 2) and 3).

Requested by @mathematicalmichael