feat: allow writing custom text to stdin on button press

[atalii]

This is just a small feature that I'll be using for my setup and notification daemon; you can merge it if you'd like :) Let me know if there's anything more to be done.

Here's the commit message explaining the change:

Rather than expecting external programs to handle 1, 2, and 3 on their standard input, this allows the user to configure the text sent themselves.

This is particularly relevant for my tool, painted, which serves as a notification daemon and takes commands to manage the notif queue over stdin. With this patch, a config like the following works just as one would expect:

[notifs]
command=painted
interval=persist

btn_left_cmd=clear
btn_right_cmd=expand

Note this only applies to bars with the persist interval, as one would expect.