zsh: pass through exit code from fzf with fzf-file-widget

This allows to have a custom widget like the following, which would
additionally accept the line, but only in case of entries being
selected:

    fzf-file-widget-with-accept() {
      zle fzf-file-widget
      if [[ "$?" == 0 ]] && (( $#BUFFER )); then
        zle accept-line
      fi
    }
    zle     -N   fzf-file-widget-with-accept
    bindkey '\e^T' fzf-file-widget-with-accept

With this `<C-a>t` will launch fzf, and simulate the pressing of "Enter"
afterwards.

Close #598

Close #596

Optimize fzf_default_completion binding

Close #587

Close #594

Add fzf_prefer_tmux option

e.g. fzf-tmux -q '$PATH'

Related: #343

Close #590

Close #583

This reverts commit f074709f.

Close #586. /cc @akashin

https://github.com/junegunn/fzf.vim/issues/147

Take SSH completion hints from known_hosts

Signed-off-by: Junegunn Choi <junegunn.c@gmail.com>

[fzf-tmux] Change tmux pane title for fzf splits

Close #580

Also extend default jump labels

Related: #555

jump and jump-accept implement EasyMotion-like movement in fzf.
Suggested by @mhrebenyuk. Close #569.