Add option to use `fzf --tmux` instead of the `fzf-tmux` script

[acristoffers]

I've described this solution in #124, so I decided to make a pull request with the solution behind an option, for those who want it. It allows to change between fzf --tmux and fzf-tmux by setting set -g @sessionx-fzf-builtin-tmux 'on'.