Embed a jupyterhub profile in nbgitpuller

There may be times when particular kinds of content requires either a different environment, hardware, etc. Since JupyterHub has support for user profiles, would it be possible to embed profile information in an nbgitpuller link.

For example, day 1 of a semester the instructor just needs a vanilla analytics environment. They send out nbgitpuller links with `profile=basic_profile` in the URL.

Mid-way through the course, the instructor starts covering advanced topics in machine learning. They need students to have access to GPUs now (which they weren't using before because of the expense). They send out nbgitpuller links with `profile=nvidia` in the URL.