Add a string functions that are runes in utf-8 strings aware

In some of my cases I have to limit length of string value from influxdb to send as a message in alert node. If string have utf-8 charset and contains some cyrillic symbols there are no way to get number of chars in it by strLength or substring it by strSubstring, etc. This happens because every cyrillic symbol counts as two chars in result of len(str).