Forward arguments set by the kernel cmdline to the init system

[Kuchteq]

Kernel can pass arguments to the init script if it encounters a double dash that should then be forwarded to the actual init system (at least that's the behavior on in other init scripts, such as ones generated by mkinitcpio).

Fixes #266

[anatol]

Thank you for the patch! It is useful to have such functionality.

It also needs

• documentation, I think https://github.com/anatol/booster/blob/master/docs/manpage.md#boot-time-kernel-parameters would be the best place to add information about parameters handling.
• tests to verify the functionality