Add cli argument to only convert new or modified ipynb files

Moving a comment from build_html.py to here:

```python
# This could be made more efficient with a cli argument to only process
# new or modified notebooks. For now it usually needs to process all,
# because the conversion script itself is changing often. But once the
# conversion process is stable, that kind of flag might be useful.
# (move this to an issue)
```

This is not a particularly high priority. For my development work, I just limit build_html.py to convert a few notebooks, and then set it to run all notebooks again.