Allow customization of fish key bindings

Before I open a pull request I just wanted to share what I did with my fzf tmux key bindings (which I originally based on this repo!) to allow for some customization.

Declare and use `_fzf_tmux_*` fish functions in fish key bindings:

https://github.com/cparadis6191/fzf-tmux.sh/commit/d1c96fb3bf60161154cd7cd300796701be266c8a

This allows for users in fish to simply redefine whichever `_fzf_tmux_*` function they like to get the customization they want. This is done by completely bypassing fzf-tmux.sh.

Next, add support for `$FZF_TMUX_DEFAULT_OPTS` so users can force `_fzf_tmux_fzf` in fzf-tmux.sh to use some different options:

https://github.com/cparadis6191/fzf-tmux.sh/commit/161b2d392f429963f04c5c0cc9942992086e10ff

This is similar in spirit to how the `fzf --bash` and `fzf --fish` key bindings work. This works in *both* bash and fish (and presumably Zsh). Users in bash can still redefine `_fzf_tmux_fzf` but now they have a new environment variable to use. And now, fish users who can't redefine the *bash* `_fzf_tmux_fzf` function can instead set `$FZF_TMUX_DEFAULT_OPTS`.