Add support for the new %w format specifier

[erikdubbelboer]

See: https://golang.org/pkg/fmt/#Errorf

package main

import (
	"fmt"
	"io"
)

func main() {
	err := fmt.Errorf("error: %w", io.EOF)
	fmt.Println(err)

}

[hairyhenderson]

This has been bugging me 😉 - @AlanQuatermain can this be merged please?

[mcristina422]

@AlanQuatermain are you still accepting PRs on this repo?