List of paths relative to the project toplevel directory that OurBigBook CLI will ignore.
Useful if your project has a large directory that does not contain OurBigBook sources, and you don't want OurBigBook to mess with it.
Only ignores recursive conversions, e.g. given:
  "ignore": [
ourbigbook .
skips that directory, but
ourbigbook web/myfile.bigb
converts it because it was explicitly requested.
TODO: also ignore during -w, --watch.