Handle share intents
This allows M.A.L.P to receive URLs from other apps (e.g. HTTP streams) and send them to MPD.
For now this only append the URL to the queue, which I think is a good default (think multiple people wanting to add their youtube video to the list), but require an extra step if MPD is not playing.
See https://developer.android.com/training/basics/intents/filters#java
PS: Awesome client, thank you very much for the work you've put into this.