Introduce a setting 'audio' in the config file to disable audio. 

This pull request adds an option to the config file, but not to the config menu yet.
In the config file you'll have added these lines by default:
```
# Enable music and sound subsystem.
audio=1
```
This will not change the games behaviour in the default, so the user will not experience any difference.

However when I was experimenting with my Raspberry pi, I noticed that the audio playback takes most of the time.
Even when the audio and sound itself were set to 0.
The difference is the initialization of the audio subsystem with ```Mix_OpenAudio``` or not.
As of now ```Mix_OpenAudio``` is always used to initialize the audio subsystem regardless if the sound is on or off. As you may change the audio settings ingame, hence we must init the audio.

However the new option can only be set offline in the config file, so we can skip audio initialization completely if it is set ```audio=0```. This disallows enabling audio in game completely.