Single notebooks and no trailing newlines

As well as a move to GitHub actions for CI (#33, #34, #35) and an improvement to the PyPI README rendering, the "tangible" changes are as follows.

Changes from #36:

• Add ability to specify the path of a single Jupyter notebook and then format only that notebook.
• Check before doing anything that the path given actually is a valid path. Add tests.

Changes from #37:

• The cell source is set to be a list where each item in the list is a line of the cell truncated with a \n except for the last line.