Use fzf for fuzzy history search

[noamross]

Would it be possible to set up radian to use fzf fuzzy search (https://github.com/junegunn/fzf) for history search directly?

Currently I can escape to shell with ;, run cat ~/radian_history | fzf, and search, but then it returns the result as an output in the shell, not at the shell prompt or R console.

Ideal implementation would allow an option() which, when turned on and fzf installed, to use fzf for Ctrl-r history search.

This is related to #147

[dshemetov]

Made a quick and hacky solution here. Still testing it, but it seems to work.