Allow non error type for the first argument of Append

[ultimaweapon]

This will have more convenience when we want to return a new simple error from a function that aggregated with the cause. Something like this:

func foo() error {
    if err := bar(); err != nil {
        return multierror.Append("failed to bar", err)
    }
}

[mitchellh]

Thanks for the PR but I don't love the ambiguity here. It isn't much effort to errors.New or fmt.Errorf (I understand it introduces an import line, but at least its just stdlib). Sorry!