Adding ALSA volume control

[noelhibbard]

This PR adds support for volume control. It adds the ---alsamixer command line option and if supplied it will control the volume. If not, volume control is disabled. The volume state is also stored and persists between sessions.

The easiest way to find the mixer name is to run alsamixer while playing some music and play around with all the sliders until you find the correct control. Take note of the name of the control at the bottom. That will be the name you would supply with the --alsamixer option.

This is my first stab at Go Lang so I'm sure my code could be streamlined. But it does work at least. :)

[AlbanSeurat]

Thanks for the contribution. some comments :

• Reading the whole confirmation at startup
• using a defer during the main fonction to dump latest status onto the config file
• Pass the volume to the player thru the dedicated go channel
• The player calling Stream.SetVolume
• Implement the Stream.SetVolume function by calling alsamixer

Would you mind changing that ? I can merge the code as-is for the sake of having the functionality and do the change myself if you prefer