Force fzf-vim to pass nohidden option to --preview-window to override shell defaults

[illia-danko]

Checklist

mainstream the latest version

Summary of Problem

I pass --preview-window=hidden to export FZF_DEFAULT_OPTS somewhere on my bashrc. However, since fzf.vim (as a dependency library used by notation-fzf-vim) backed with the global FZF_DEFAULT_OPTS, it's no way with a current implementation to explicitly set --preview-window=nohidden to override the defaults when hidden is set.

@alok Do you consider to use nohidden value, eg. let s:show_preview = get(g:, 'nv_show_preview', 1) ? 'nohidden' : 'hidden' as a potential fix of the issue?