Respect nREPL's config file

[bbatsov]

nREPL 0.5 introduced some configuration files, which ideally should be respected by REPLs using nREPL. (see https://nrepl.org/nrepl/0.6.0/usage/server.html#_server_configuration)

Lein and clj already honour the config file, and it be nice if boot did the same.

I guess this ticket is mostly a reminder for myself to implement this, but if someone wants to tackle it - it's a pretty simple and straightforward task.