Added quoted tweets to conversation command

[nealchandra]

This PR updates the conversation command to include quoted tweets in the conversation history instead of replies only. Many users tend to use quotes as an alternative way to reply to/comment on a thread, so conversation seemed like the appropriate command to piggyback on. I noticed a few folks had requested this in orakaro/rainbowstream#227, and the change was fairly straightforward so I thought putting it in a PR might be useful to others.