Add option to specify extra free space in pmbootstrap init

This #1904 (closed) and adds a new commandline flag -E / --extra-space for specifying the amount of additional space to be added tothe image size to work around cases where the automatically determined size turns out to not actually be enough.

The value is also asked for in the "Additional options"section of the interactive mode.

This is my first ever merge request for pmbootstrap so I probably did a few things wrong. Happy to receive feedback. :)