Adding WroteHelp() helper

[iamgnat]

Adding a helper function to simplify the Parse*() caller determining if the help text was written or not.

Usage would look like:

...
_, err := flags.Parse(opts)
if flags.WroteHelp(err) {
    os.Exit(0)
}
if err != nil {
    ...
}
...

Test output:

$ go test -v -run TestWroteHelp
=== RUN   TestWroteHelp
=== RUN   TestWroteHelp/No_error
=== RUN   TestWroteHelp/Plain_error
=== RUN   TestWroteHelp/ErrUnknown
=== RUN   TestWroteHelp/ErrHelp
--- PASS: TestWroteHelp (0.00s)
    --- PASS: TestWroteHelp/No_error (0.00s)
    --- PASS: TestWroteHelp/Plain_error (0.00s)
    --- PASS: TestWroteHelp/ErrUnknown (0.00s)
    --- PASS: TestWroteHelp/ErrHelp (0.00s)
PASS
ok  	github.com/jessevdk/go-flags	0.002s

[iamgnat]

@jessevdk Can I please have a review of this PR?