Add support for plain text output

Thank you so much for this handy module. I love all the trim options provided, but often need a plain text version of the output that is also smartly trimmed. I found this other issue along the same lines: ✨ Add formatter for trimmed plain text Closed: outdated

I was thinking about something a little simpler though, perhaps the addition of a single checkbox into the existing smart trimmed formatter. If that box is checked, then check_plain() is run on the output.