gpio: add short analog (bbb) example

samples/.gitignore

@@ -1,4 +1,5 @@
 analog
+analogshort
 bh1750fvi
 bmp085
 bmp180

samples/analogshort.go

@@ -0,0 +1,17 @@
+// +build ignore
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/kidoman/embd"
+)
+
+func main() {
+	embd.InitGPIO()
+	defer embd.CloseGPIO()
+
+	val, _ := embd.AnalogRead(0)
+	fmt.Printf("Reading: %v\n", val)
+}