Define x11-repl command that optionally takes the name of an interpreter
to start it on a new window. Users then can select some text in the
editor and use [x11-]send-text command to send it to the interpreter.
Ideally the latter command should be bound to a key for easier
interaction. Requires xsel and xdotool to work.
Echo supported switches are not well defined, so echo usage that
directly pass a shell variable as parameter may not have the
expected behaviour. Using printf %s should be safer.
