Allow periods in tags
diff --git a/tag.go b/tag.go
index bbb3068..0382276 100644
--- a/tag.go
+++ b/tag.go
@@ -43,7 +43,7 @@
 		return false
 	}
 	for _, c := range key {
-		if c != ' ' && c != '$' && c != '-' && c != '_' && !unicode.IsLetter(c) && !unicode.IsDigit(c) {
+		if c != ' ' && c != '$' && c != '-' && c != '_' && c != '.' && !unicode.IsLetter(c) && !unicode.IsDigit(c) {
 			return false
 		}
 	}