replace some basic uses of fmt.Sprintf()

Really tiny gains here, and doesn't significantly impact readability:

    BenchmarkSprintf
    BenchmarkSprintf-10    11528700     91.59 ns/op   32 B/op  1 allocs/op
    BenchmarkConcat
    BenchmarkConcat-10    100000000     11.76 ns/op    0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/events.go

@@ -133,7 +133,7 @@ func prettyPrintEvent(out io.Writer, event eventtypes.Message) error {
 		sort.Strings(keys)
 		for _, k := range keys {
 			v := event.Actor.Attributes[k]
-			attrs = append(attrs, fmt.Sprintf("%s=%s", k, v))
+			attrs = append(attrs, k+"="+v)
 		}
 		fmt.Fprintf(out, " (%s)", strings.Join(attrs, ", "))
 	}