Added flags.Help and flags.PrintHelp methods

This pull requests add two helper methods: `flags.Help()` and `flags.PrintHelp()`. The first one returns the help message as a string, while the second one prints it directly to stdout (similar to `flag.PrintDefault()` from the standard package). `flags.Parse` or `flags.ParseArgs` must be called before calling any of these two new methods.

Since go-flags did not keep track of the parser created by `flag.Parse` (since it wasn't needed), I've added a new `defaultParser` variable which will hold the last parser created by these two functions.