Add NewPtrOr

NewPtrOr allows you to fall back to a concrete value if the pointer was nil.

d := NewPtrOr(description, "(no description)")
// if "description" was set, d contains the value
// if "description" was nil, d "(no description)"

Full Changelog: v0.6.0...v0.6.1

Add curried variations for mapping functions

These allow you to more ergonomically perform mapping operations on many values.