Wrap, Wrapf equivalents that return nil on nil input

[matthewleon]

It might be useful to have Wrap and Wrapf equivalents that return nil when given nil input, allowing the called to avoid preceding these calls with a nil check. Would you accept a PR for this?

[matthewleon]

In fact, the tryOpen example you give in your README would be simpler using one of these functions.

[mitchellh]

Yes, that sounds very good to me.