Add XFF/Forwarded custom filter strategy?

[adam-p]

It's possible that a user of the library may wish to use our X-Forwarded-For and Forwarded handling/parsers, but choose which IP to use in a different way (than "rightmost non-private", "trusted count", etc.). So they would like strategies like:

func NewLeftmostCustomStrategy(headerName string, filter func(ip string) bool) ...

func NewRightmostCustomStrategy(headerName string, filter func(ip string) bool) ...

Those would choose the left/rightmost IP for which filter returns true.

I don't want to add this just because someone "might" want them, but if anyone has any solid examples/use cases, please let us know here.

If we do add this, we might want to also export privateAndLocalRanges and/or isPrivateOrLocal. Those are likely to be needed by a custom filter. (Possibly other helpers, like goodIPAddr?)