Split large conversations into multiple files

[GjjvdBurg]

Suggested by @ma-phi via email. For long conversations it might be nice to optionally separate conversations into multiple HTML files such that they load faster when viewed in the browser. If we do this it would be helpful to show the start date in the filename.

A simple way to do this would be to add a --max-msg or --thread-length flag to the UI and split the messages into chunks just before we write to HTML.