Isolate `nrepl-client` connection logic from CIDER

Hi Bozhidar,

I have just writen  a [nREPL client and server for R](https://github.com/vspinu/R-nREPL/tree/master/) and with this patch I can sucesfuly [connect/jack-in](https://github.com/vspinu/edamot/blob/master/edamot.el). There are a bunch of other, less critical, cider links that must be broken. I will be addressing those gradually. 

I hope you would be fine with spliting nrepl into a separte package eventually. 

Thanks.

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Allow `nrepl-client` to be used by non CIDER frontends.

- Frontends can control the creation of the client buffer through
  `nrepl-create-client-buffer-function`.

- Each frontend has it's own `xxx-connected-hook` which is run by
  `xxx--connected-handler`. The latter function is placed in the local
  `nrepl-connected-hook` during the client buffer initialization. Similarly for
  `xxx-disconnected-hook`.

- Rename `nrepl--init-connection-buffer` -> nrepl--init-capabilities
> These are missing package-version properties.

I wonder if the `defcustom` is really needed here. I added them because defcustom was used for`nrepl-connection-hook`.