Add argument for download-latest-event-logs.py

[MarcDoughty]

I'm writing a script to grab these logs to a container and pipe them up to a cloud logging platform. In an effort to keep per-customer code and load on you minimal, I think it would be handy to be able to pass an argument to download-latest-event-logs.py for number of previous logs to download (currently hard-coded at 5).

Should I do a PR for that, or a PR to pass a 'retentionhours' argument that triggered cleanup of old logs? If not, I can reference this project in the wrapper I'm writing in my own repo and this can be a placeholder. Not sure if you want an 'inside-out' or 'outside-in' model for this type of thing.