Could regional_summary have an option to turn off plot generation?

[RichardMN]

This is a trial balloon for a PR.

My personal use case for EpiNow2 works with @hamishgibbs RtD3 framework using just the CSVs generated by regional_summary. I delete the generated graphs (sorry), when I remember to clean up. Would it make sense to put in a param to regional_summary which adds an additional test at line 278 and skips until line 349 of summarise.R?