Proposal: add option to change delimiter

[tamiraviv]

When using errors.Wrap the default delimiter is ": ".
But for me at least it seem like it should be "| ", with that I can wrap errors like this:

errors.Wrapf(err, "Error while trying to open file: %s", fileName)

I thing that instead of hard-coded delimiter we should be able to configure it with:

errors.SetDelimiter (string)