Add validation for region/platform

ovrstat.go

@@ -57,4 +57,12 @@ func (s *OvrStatApi) FetchHeroStats(platform, region, battletag string, heroes [
 	}
 
 	return &r, nil
+}
+
+func ValidPlatform(platform string) bool {
+	return platform == "pc" || platform == "xbl" || platform == "psn"
+}
+
+func ValidRegion(region string) bool {
+	return region == "us" || region == "eu" || region == "kr"
 }