Add dynamic mixer example (#467)

This example plays a C Major 6th chord by adding four sources, sine waves frequencies of C, E, G, and A, to a `DynamicMixerController`, and appends its respective `DynamicMixer` to a sink.

examples/mix_multiple_sources.rs

@@ -0,0 +1,38 @@
+use rodio::source::{SineWave, Source};
+use rodio::{dynamic_mixer, OutputStream, Sink};
+use std::time::Duration;
+
+fn main() {
+    // Construct a dynamic controller and mixer, stream_handle, and sink.
+    let (controller, mixer) = dynamic_mixer::mixer::<f32>(2, 44_100);
+    let (_stream, stream_handle) = OutputStream::try_default().unwrap();
+    let sink = Sink::try_new(&stream_handle).unwrap();
+
+    // Create four unique sources. The frequencies used here correspond
+    // notes in the key of C and in octave 4: C4, or middle C on a piano,
+    // E4, G4, and A4 respectively.
+    let source_c = SineWave::new(261.63)
+        .take_duration(Duration::from_secs_f32(1.))
+        .amplify(0.20);
+    let source_e = SineWave::new(329.63)
+        .take_duration(Duration::from_secs_f32(1.))
+        .amplify(0.20);
+    let source_g = SineWave::new(392.0)
+        .take_duration(Duration::from_secs_f32(1.))
+        .amplify(0.20);
+    let source_a = SineWave::new(440.0)
+        .take_duration(Duration::from_secs_f32(1.))
+        .amplify(0.20);
+
+    // Add sources C, E, G, and A to the mixer controller.
+    controller.add(source_c);
+    controller.add(source_e);
+    controller.add(source_g);
+    controller.add(source_a);
+
+    // Append the dynamic mixer to the sink to play a C major 6th chord.
+    sink.append(mixer);
+
+    // Sleep the thread until sink is empty.
+    sink.sleep_until_end();
+}