Fix trailing arguments being considered errant positionals

[integrii]

Fixes #79

If you would like to use a variable number of arguments at the end of your program without the user specifying -- before the list, you can do this by setting flaggy.ShowHelpOnUnexpectedDisable() and then parsing the []string at flaggy.TrailingArguments.

• new trailingArguments example
• new tests for trailing arguments with positionals. fixed trailing arguments being incorrectly considered as positionals