FlagSet.Parse behaviour changed between v1.0.0 and v1.0.1

[dnephin]

Somewhere between these 2 releases there was a change in behaviour.

In v1.0.0 an invalid flag causes flag.Parse() to print Usage and print the error (unknown shorthand flag...), as well as return the error.

In v1.0.1 an invalid flag causes flag.Parse() to return the error, but print nothing (which makes it very confusing for a user because there is no error printed anymore).

I'm using flags.ContinueOnError and flags.SetInterspersed(false), but I'm not sure if it's specific to either of those.

I think the change comes from #138. Although the diff seems like it's doing the opposite.

I actually like the new behaviour better, but the change was surprising from 1.0.0 to v1.0.1 without any mention of it in the release notes.