Enhance `cider-connect` to auto-detect running nREPL servers

 - `cider-connect` now interactively asks for host from a combined list of hosts
   from machine-wide ssh configuration and `cider-known-endpoints`.

 - Once the host has been selected `cider-connect` locates ports of the running
   instances of nREPL servers on the corresponding host (currently only those
   started with lein repl). The machine must have `ps` and `grep` programs in
   the bin path.