Add support for --tmux-title option

[afh]

Personally I see value in adding support for a --tmux-title option, with which a title for the tmux popup can be set (see -T option of display-popup command in tmux(1)). This option would only work in combination with the --tmux option and look better if the --no-border option is set as to avoid rendering two borders: one from fzf and one from tmux, rendering the tmux border is required in order to display the title.

Please note that this is just a minimal working proof-of-concept. If the maintainers and folks using fzf find this useful and would consider this for inclusion, I'd be happy to elaborate on this PR; ideally with some guidance from the maintainers given that I'm rather unfamiliar with fzf's codebase.

[junegunn]

I would suggest that you use --border-label instead. You can configure the position of the label using --border-label-pos and it also supports ANSI color codes. Do you think you still need to use -T option?

fzf --tmux --border-label ' Hello '

fzf --tmux --border-label ' Hello ' --border-label-pos 2

fzf --tmux --border-label $' \x1b[33mHello '

[afh]

Thank you for the quick and helpful response, @junegunn, very much appreciated! Exactly what I'm looking for; not sure how I missed this when skimming through fzf(1) while looking for title.