Re-organise code to facilitate future improvements

[arobrien]

Separate concerns and reduce duplicate code by creating methods.

This will better facilitate future improvements to make nbstripout more flexible, such as:

• making configuration easier to manage to facilitate reading from other sources such as tox.ini or pyproject.toml
• create a --check mode
• running nbstripout as part of a ci pipeline

Doing a dummpy PR in my own fork for now, while I get the hang of this project.