Single dash long options if no shorthand defined.

[b1v1r]

I have had issues migrating from https://github.com/koding/multiconfig or other https://golang.org/pkg/flag/ based configuration to pflag. This is because existing users were using the long options with a single dash (-option vs --option). The pflag package does not seem to support this and just errors trying to interpret these as shorthand options (e.g., -option => -o -p -t -i -o -n).

To allow easier migration when not using the shorthand option feature, I made a small change to interpret single dash options as double dash options when no shorthand options are defined. This seemed the least intrusive and did not seem to mess up anyone using shorthand options while truly making this a drop-in replacement for the flag package which it was not really before due to this.

[b1v1r]

Just a ping on this. Any comments?

[moorereason]

Your PR literally breaks the whole point of this package, and I can't imagine why the maintainers would merge it. Drop-in replacement for the flag package is talking about the API, not the argument parsing algorithm.

[stevenroose]

I agree with @moorereason that this is not a desired feature.

[NDagestad]

Digging up a very old and closed PR to say that the README should be updated.
It explicitly states that

pflag is a drop-in replacement of Go's native flag package. If you import pflag under the name "flag" then all code should continue to function with no changes.

Which it clearly does not since options aren't parsed as they were with flag.
I am not saying pflag need to support this, but things need to be clear.

Edit:
And to moorereason the readme states that the package is a drop in replacement adding POSIX style --options, not that it follows the POSIX behaviour only and rigorously