Add an Append function for easier usage of merror

[noxer]

This PR adds an Append function to the package itself. It checks if the first argument is already a *MError and turns it into one if not. It also ignores nil errors which allows very easy usage in loops.

var err error
for _, thing := range things {
    err = merror.Append(err, thing.Do())
}

[wiggin77]

Thanks @noxer. This is merged and a release published. You'll need to upgrade to v1.0.4