This will allow users to customize the prefix section or env hint
section of the flag entries in the help menu without having to
reimplement the rest of the logic required in defining FlagStringer.
by defining `cli.DefaultFlagStringFunc` with a default value that uses
`withEnvHint`, conditionally running a given flag's `FormatValueHelp` if
present.
Closes#257
Now the exit code can be returned by BeforeFn, ActionFn and AfterFn.
The `os.Exit` function is not called by this packaged
This closes#66 and closes#164
rliebz
Matt Farina
Dan Buch
Tarcísio Gruppi