This keeps the user's session clean by not using it for things the user
did not directly dispatch (such as omnicomplete). On the fence but
currently included in the user session is commands like :Doc.
A bug in the current version of nREPL doesn't send us *out* and *err* if
a session is used. nREPL 0.2.0-RC2 promises to fix that. When it does,
we'll start using sessions.
Closes#11.