Add support for end-of-options delimiter

[DannyBen]

cc #734

Adds support for using -- as an end-of-options delimiter. For commands that
define positional arguments, values after the delimiter are parsed as positional
arguments even when they start with -.

Commands with catch_all retain their existing special behavior: everything
after -- is stored in other_args, even when an optional positional argument
is still available.

Also adds an end-of-options-delimiter example demonstrating the new behavior.

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