Add a way to know the state of --dnd flag

[welcoMattic]

Following #80 and #99, could we consider adding an way to know the state of the --dnd flag?

I'm using i3 + i3status-rust, with a toggle block to on/off notification. This block needs to know the actual state of dnd to display the right icon.

[lukelex]

Can this be done in the near future?