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

Following #80 and #99, could we consider adding an way to know the state of the --dnd flag? 

I'm using i3 + [i3status-rust](https://github.com/greshake/i3status-rust), with a [toggle block](https://github.com/greshake/i3status-rust/blob/v0.22.0/doc/blocks.md#toggle) to on/off notification. This block needs to know the actual state of dnd to display the right icon.