Add desktop notification on playing new track

[rbuch]

Addresses #202.

[rbuch]

This is my first time doing anything with Rust, so please review with a critical eye.

[hrkfdn]

Thanks! I have merged this with some minor changes.

[pfr-dev]

Can anyone help me configure notifications? I've added notify = true to my config.toml but I need to know how to use it with something like dunst or xnotify on a wm only environment. I guess I need a script to pipe it to stdout or something but It's a bit over my head.

[BachoSeven]

@Ramiferous For dunst, I've set it up like this:

[ncspot]
	appname = *ncspot*
	format = "<b> Now Playing:</b>\n%s\n%b"
	background = "#1d2021"
	foreground = "#ebdbb2"
	frame_color = "#1db954"
	timeout = 5
	script = spotup

Edit: spotup is just a script of mine which updates a statusbar module.