Allow to copy path to element to arbitrary clipboard tool

[bew]

AFAIK The current clipboard system only works on Linux and MacOS. It does not support a number of use cases like connecting to a server via ssh (without a running desktop environment, no X/Wayland/..) and running jless there, wanting to copy the path of an element.

Related to #10 #47 #50

With #84 clipboard can be completely optional, and jless could be configurable (via cli args? Like --clipboard-cmd ?) to set the command to run when trying to copy the path.

It could be used to copy to a tmux buffer, or to pass the data to OSC52 directly to the host terminal (to work on any host, through any ssh connections)