Add exported functions to preserve pkg/flag compatibility

[mckern]

These changes bring behavior inline with go's flag library, and allows for printing output directly to whatever the current FlagSet is using for output and retrieving a given FlagSet's name. These changes will make it easier to correctly emit output to stdout or stderr (e.g. a user has requested a help screen, which should emit to stdout since it's the desired outcome).

(This PR has also pulled in #241, since @5paceToast and I were clearly working in the same direction here)

example for Out():

import (
  "fmt"
  flag "github.com/spf13/pflag"
)

func init() {
  var helpFlag bool
  whoami := path.Base(os.Args[0])
  flag.BoolVarP(&helpFlag, "help", "h", false, "show this help")

  flag.Usage = func() {
    fmt.Fprintf(flag.CommandLine.Output(), "%s: print example help\n\n", whoami)
    fmt.Fprintf(flag.CommandLine.Output(), "usage: %s [-h]\n", whoami)
    flag.PrintDefaults()
  }

  flag.Parse()

  if helpFlag {
    flag.CommandLine.SetOutput(os.Stdout)
    flag.Usage()
    os.Exit(0)
  }
}

example for Name():

import (
  "fmt"
  flag "github.com/spf13/pflag"
)

func init() {
  versionNumber := "123.456.789"
  myFlagSet := flag.NewFlagSet("Megumin", flag.ExitOnError)

  fmt.Printf("%s\t%s\n", myFlagSet.Name(), versionNumber)
}

[CLAassistant]

All committers have signed the CLA.

[CosmicToast]

I'd prefer if the testName was capitalized (as is the standard, I'm not sure it would even run in this state) -> TestName.
Besides that LGTM!

[mckern]

Oh it definitely runs (I induced a failure state to check). It doesn't look like the standard is consistently applied and I'm not sure why some tests end up as public functions and some don't:

ryan@bender pflag (git:output) $ ag -c -s -Q 'func Test' flag_test.go
35
ryan@bender pflag (git:output) $ ag -c -s -Q 'func test' flag_test.go
5

Looks like more of them are public than not though, so yeah, let's go with that.

[mckern]

/cc @eparis I think we've worked it out, if you're still available for review? Please & thank you!

[mckern]

Ping @eparis ?

[mckern]

Ping @eparis and/or @therealmitchconnors ?

[wojtekk-ac]

just 3cents, I am kind of waiting for that to be in pflag since literally years. (had sibling issue #230 opened here)

[wojtekk-ac]

Thank you @therealmitchconnors !

[mckern]

thanks @therealmitchconnors 🎉 !