snf-mkimage: Add --host-run option

This can be used to define scripts to run on the guest media after
mounting it under a directory in the host.

This resolves #13, resolves #14 and resolves #15
Add support list sysprep parameters

The type for those parameters is list:<type>
Make the sensitive_userdata a list sysprep param

The user may alter the list of sensitive userdata
Fill the KERNEL metadata for Arch Linux