Kingpin is a fluent-style, type-safe command-line parser. It supports flags,
nested commands, and positional arguments.

Features:
+ Help output that isn't as ugly as sin.
+ Fully customisable help, via Go templates.
+ Parsed, type-safe flags (kingpin.Flag("f", "help").Int())
+ Parsed, type-safe positional arguments (kingpin.Arg("a", "help").Int()).
+ Parsed, type-safe, arbitrarily deep commands (kingpin.Command("c", "help")).
+ Support for required flags and required positional arguments (kingpin.Flag("f", "").Required().Int()).
+ Support for arbitrarily nested default commands (command.Default()).
+ Callbacks per command, flag and argument (kingpin.Command("c", "").Action(myAction)).
+ POSIX-style short flag combining (-a -b -> -ab).
+ Short-flag+parameter combining (-a parm -> -aparm).
+ Read command-line from files (@).
+ Automatically generate man pages (--help-man).