New customization variable: nrepl-popup-stacktraces-in-repl

We use nrepl-popup-stacktraces only non-REPL buffers, and introduce a
specific flag for the latter.
CHANGELOG and README entries for nrepl-popup-stacktraces-in-repl
Merge pull request #259 from jaor/local-popup

Allow local values for nrepl-popup-stacktraces