raise_on_error doesn't work as expected.

[asaff1]

Hello, First, this is a nice library! very clean API.

Some suggestions for improvement:

1. I would expect raise_on_error to throw when unrecognized argument is provided. (Similar to how python argparse behaves) Instead the library simply prints a message "unrecognized argument" and moves on. Also no flag is returned that the user can act a upon (maybe a subclass hook). This may lead to typos that the user won't notice.

2. Can be nice if the library can accept both --my-value and --my_value (also done by Python argparse)