multi_if solution and benchmark

multi_if.bench

@@ -0,0 +1,3 @@
+PASS
+BenchmarkConvert-8	 2000000	       741 ns/op
+ok  	exercism/go/raindrops	2.253s

raindrops.go

@@ -2,8 +2,25 @@
 
 package raindrops
 
+import "strconv"
+
 const testVersion = 2
 
-func Convert(int) string
+func Convert(i int) string {
+	var s string
+	if i%3 == 0 {
+		s += "Pling"
+	}
+	if i%5 == 0 {
+		s += "Plang"
+	}
+	if i%7 == 0 {
+		s += "Plong"
+	}
+	if s == "" {
+		s = strconv.Itoa(i)
+	}
+	return s
+}
 
 // The test program has a benchmark too.  How fast does your Convert convert?