grep needs regexp support

[rxgrant]

Current conversation grep does not support -E regexps.

[JeanMertz]

Agreed. I haven't had a need for regexp, but it makes sense to add it.

I should note that I also have a draft RFD locally that investigates the possibility of a custom filtering DSL for conversations, which would allow more advanced searching and filtering based on turn numbers, distinctions between request/response messages, etc. But as I said, still in draft, we can already get a quick win by supporting -E.