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

[tagobar]

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).

[leonty]

Same problem. In string functions strings must be considered as a set of characters, not bytes.

[sazonov]

@tagobar thanks for reporting. Same problem here.

[darkhorse77]

Same problem

[dmitry-worker]

+1

[brainstormka]

+1

[SkeletonDarkknight]

have same problem. please solve it.