(TK-11) Make custom middlewares possible in the nrepl-service

With this change, one can enable custom middlewares for the nrep.
For example to make the LightTable nrepl connection work:
Add [lein-light-nrepl "0.0.4"] to your project dependencies.
Configure your nrepl-service like this:

[nrepl]
port = 12345
enabled = true
middlewares = [lighttable.nrepl.handler/lighttable-ops]