Support building single notebook(s)

It's useful to have a way to process a single notebook, or maybe a few... there could be two approaches.

- Make a way to pass a list of .ipynb files and get processed files (where? how named?)
- OR, make almost everything in the course YAML file optional.

Currently, option 2 would need (I think) `title` and `extras` to be optional, see #26 