Better control over nREPL starting [IMMUTANT-416]

[:ring :nrepl] overrides [:immutant :nrepl-port] now. If the former
exists, then the latter will be ignored, so the value of [:ring :nrepl
:start?] determines whether a repl will be started. If [:ring :nrepl]
doesn't exist, then [:immutant :nrepl-port] will be used. If not set, it
will default to 0 when the :dev profile is active. If explicitly set to
nil, no repl will be started.