Allow applications other than classic Notebook to open on server

[mdpiper]

Currently, nbgitpuller-link is only configured to open files on a JupyterHub using the classic Jupyter Notebook interface. However, other applications, such as JupyterLab, can can be opened with nbgitpuller, as described in the documentation.

This is a request to support other JupyterHub interfaces in nbgitpuller-link; at minimum, supporting the JupyterLab interface.

[mdpiper]

Leaving this issue open (after merging #2) because the Shiny and RStudio interfaces haven't been implemented.