Feature request: Configuration files

[codesections]

mn is getting more powerful, and we should let users take advantage of that power with custom configurations (rather than by passing command-line flags).

Ideally, we would have both global config values and per-file config values set in frontmatter.

These values should be configurable from the command line with syntax similar to git config

[codesections]

#17 took the first steps towards implementing this. Global config is now supported; front matter and editing through the command line are still WIP