Allow local values for nrepl-popup-stacktraces

With this change, one can have buffer-local values for this variable
and, for instance, avoid popups in the REPL while maintaining them for
evaluations in clojure files.

I add a hook to nrepl-mode-hook that uses `(set (make-local-variable ..`

An alternative, and perhaps simpler, way would be to provide separate
flags for REPL and Clojure buffers.