Going off of how Toby's last forum project went[1], most things that aren't essential to the project are left open to implement as plugins. esoTalk does have a plugin that lets conversation authors mark one reply as an answer, so I think it's reasonable to guess that one will eventually appear for Flarum.
We built Flarum to be highly extensible, so something like this could easily be implemented! Eventually we'll probably make an official Q&A extension too. If you want to learn more about Flarum's extensibility, check out http://flarum.org/docs/extend/.
