Convert int to string using rune() (#66)

See https://github.com/golang/go/issues/32479

Fix #65.

Signed-off-by: Robert-André Mauchin <zebob.m@gmail.com>

formatter.go

diff --git a/formatter.go b/formatter.go
index df61d8d..bf4b598 100644
--- a/formatter.go
+++ b/formatter.go
@@ -37,7 +37,7 @@
 	s := "%"
 	for i := 0; i < 128; i++ {
 		if f.Flag(i) {
-			s += string(i)
+			s += string(rune(i))
 		}
 	}
 	if w, ok := f.Width(); ok {