add fzf-tmux support

[denn-moe]

hey,

thank you for making such a great tool. this is my first pull request ever, so please be gentle.

fzf ships with with a wrapper-script for tmux integration. fzf-tmux -p opens a popup windows in tmux. i added the ability to define your own "fzf command" when you set the variable fifc_fzf_cmd to "fzf-tmux -p 80%,60%" for example like: set -Ux fifc_fzf_cmd "fzf-tmux -p 80%,60%" the completion interface will be shown in a popup window.

this is my first ever contribution to an open source project and i really suck at variable names... =] i didn't want to mess with your readme for my little change.

i'd like to hear some criticism, got a lot to learn.

cheers