Example application using the mixer API

[zmarvel]

• Example application using mixer API
• Added one sound effect and a looping chiptune to examples/resources, with credits

On Debian 10, running this example depends on #143.

[a-hurst]

This looks great! This will be very helpful for newcomers trying to wrap their heads around SDL2_mixer's arcane audio API.

Two requests before I merge this:

• First, can you make the coin noise louder relative to the music? It's a little hard to hear on my (admittedly terrible) ThinkPad speakers, but it's an important part of the demo. Adjusting the volume of either the music or coin noise would also be a helpful illustration for the example, too.
• Second, would it be possible to compress the music as .ogg and have the coin noise as the .WAV example instead? I only ask because including it adds 2MB of bulk to a repository that's only 3.5MB total to begin with, and I'd like to keep things lean.

Thanks again for contributing!