Remove \r from the special chars list

Bash doesn't actually interpret \r specially at all, so remove it from
the list of characters that need escaping.
diff --git a/quote.go b/quote.go
index 2cfa912..f45f198 100644
--- a/quote.go
+++ b/quote.go
@@ -22,7 +22,7 @@
 
 const (
 	specialChars      = "\\'\"`${[|&;<>()*?!"
-	extraSpecialChars = " \t\r\n"
+	extraSpecialChars = " \t\n"
 	prefixChars       = "~"
 )