[fzf-tmux] Pass $BAT_THEME

This may anger some purists, but bat is widely used as the previewer so
I think it's worth it.

bin/fzf-tmux

@@ -192,6 +192,7 @@ if [[ "$opt" =~ "-E" ]]; then
 fi
 [[ -n "$FZF_DEFAULT_OPTS"    ]] && envs="$envs FZF_DEFAULT_OPTS=$(printf %q "$FZF_DEFAULT_OPTS")"
 [[ -n "$FZF_DEFAULT_COMMAND" ]] && envs="$envs FZF_DEFAULT_COMMAND=$(printf %q "$FZF_DEFAULT_COMMAND")"
+[[ -n "$BAT_THEME" ]] && envs="$envs BAT_THEME=$(printf %q "$BAT_THEME")"
 echo "$envs;" > "$argsf"
 
 # Build arguments to fzf