Use ConfigManager for configuration options

[bollwyvl]

As part of #428, it will likely become desirable to use the ConfigManager, a la IPython.html.notebookapp to configure nbviewer.

Specifically, this will make specifying and configuring modular, potentially un-bundled providers more tractable.

Generally, this will help in having a single place to document nbviewer-specific configuration options, and configuring upstream components like nbconvert, and make it easier for an in-house deployment to include specific information, such as #170, templates and other branding issues.

[krinsman]

Is this related to #840? Or is the type of configurability that JupyterHub uses (implemented in that PR) different from the notebook's ConfigManager? (I'm not familiar with that.)

It doesn't seem the same, but it also seems like ConfigManager requires a configurable class to work with, and that PR seems to be a first step towards making NBViewer configurable.

Several commits after that PR I was able to use such an nbviewer_config.py file to implement a "Notebook cloning extension" for NBViewer, so I think it works and is useful for the reasons you mentioned. Again, it's basically copied from what JupyterHub does.