The Errors function is not related to diag.Error, so keeping it in the same
package is confusing.
Also unexport the MultiError type. This allows its Error method to be
simplified, as it can assume that it always contains at least 2 errors.
- Don't require creating a Getopt object in the API.
- Add a new Parse function, and rename the existing method to Complete.
- Add an Unknown field to Option to indicate unknown options.
- Rewrite the tests.
- Numerous stylistic changes.
