Api function to reload all tym instances

[Stardust-kyun]

As far as I can tell from the docs, tym does not have an api function that can reload all running instances of tym. This would allow for changes to the config or theme to be made without needing to go through and reload every instance manually.

[endaaman]

A combination of get_ids API and signals might help.

tym.set_keymap('<Ctrl>0', function()
  for id in pairs(tym.get_ids()) do
    tym.signal(id, 'hook', {'reload'})
  end
end)

tym.set_hook('signal', function(command)
  if command == 'reload' then
    tym.reload()
    return
  end
end)

Also added some docs to README about these IPC stuffs ( 5574e86 ), thanks.