Draft: Big ol' refactor

This is a huge refactor, I don't think anyone will review any or all of this, so I'm going to work on writing more tests before merging it.

A lot of how mkinitfs is configured is changed in this series, many of the changes are breaking changes. The next release of mkinitfs with these patches should be tagged 2.0.

fixes: #8 (closed) fixes: #3 (closed)