Fixes # 44

internal/cmdflag/cmdflag.go

@@ -21,7 +21,7 @@ const VersionBoolFlag = "version"
 // Usage is the function used for help.
 var Usage = func() {
 	fset := flag.CommandLine
-	out := fset.Output()
+	out := fsetOutput(fset)
 
 	program := programName(os.Args[0])
 	fmt.Fprintf(out, "Usage of %s:\n", program)

internal/cmdflag/fset.go

@@ -0,0 +1,12 @@
+// +build go1.10
+
+package cmdflag
+
+import (
+	"flag"
+	"io"
+)
+
+func fsetOutput(fs *flag.FlagSet) io.Writer {
+	return fs.Output()
+}

internal/cmdflag/fset_1_10.go

@@ -0,0 +1,13 @@
+// +build !go1.10
+
+package cmdflag
+
+import (
+	"flag"
+	"io"
+	"os"
+)
+
+func fsetOutput(fs *flag.FlagSet) io.Writer {
+	return os.Stderr
+}