Define tmux-repl-vertical/horizontal/window commands that optionally
takes the name of an interpreter to start it on a new pane/window. Users
then can select some text in the editor and use [tmux-]send-text command
to send it to the interpreter. Ideally the latter command should be
bound to a key for easier interaction.
This commit adds a kak script dedicated to tmux support, and declares
new command to interact with the current tmux session. Users are now
able to create vertical panes or horizontal panes arbitrarily (as
opposed to setting the `termcmd` variable to the proper command). The
script also has its own `focus` function, used to jump from a pane to
another.
