Add midi playback example

examples/midi/Buy to the Beat - V2 License.md

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+# Buy to the Beat - V2
+
+Copyright (c) 2021 Robert Oost<br>
+Soundcloud: https://soundcloud.com/sponsrob
+
+This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) License. To view a copy of the license, visit https://creativecommons.org/licenses/by-nc-sa/4.0/.

examples/midi/Florestan-Basic License.md

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+# Florestan Basic GM GS
+
+Author: Nando Florestan
+
+This sound font is in the public domain.
+
+Source: [Internet Archive](https://archive.org/details/sf2-soundfonts-free-use)

examples/midi/main.go

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+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"github.com/gopxl/beep"
+	"github.com/gopxl/beep/midi"
+	"github.com/gopxl/beep/speaker"
+)
+
+func main() {
+	var sampleRate beep.SampleRate = 44100
+
+	err := speaker.Init(sampleRate, sampleRate.N(time.Second/30))
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Load a soundfont.
+	soundFontFile, err := os.Open("Florestan-Basic-GM-GS-by-Nando-Florestan(Public-Domain).sf2")
+	if err != nil {
+		log.Fatal(err)
+	}
+	soundFont, err := midi.NewSoundFont(soundFontFile)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Load a midi track.
+	midiFile, err := os.Open("Buy to the Beat - V2.mid")
+	if err != nil {
+		log.Fatal(err)
+	}
+	s, format, err := midi.Decode(midiFile, soundFont, sampleRate)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("Song duration: %v\n", format.SampleRate.D(s.Len()))
+	speaker.PlayAndWait(s)
+}