Add Feature: Select a function from ns with IDO and insert into REPL buffer

Interested to see if this is wanted as a feature, something I use and others have found useful. To try out, put some fns in user.clj and go to the repl and do C-c M-f, it'll help you choose a fn using ido and insert it into the repl buffer ready to run.

This is helpful as people often have utility fns they want to use regularly. ac-nrepl is close to save on the typing, but this PR only pulls back functions whereas ac-nepl does all vars. Also using ido is nicer to give you a 'browsing' experience, like 'what fns can I run in this ns?' Finally this PR filters out clojure core fns (which could be configurable).