Add a new module that contains a utilities to inspect an initrd.
It contains a `Initrd` class that loads a given initrd and can
then be used get the list of files via Initrd.filelist, the dracut
modules via `Initrd.modules` and the kernel modules via the
`Initrd.kmods` properties. Another top level function `read_initrd`
returns a dict that contains the name of the initrd as key and as
value another dictionary with `modules`, `kmods` as well as the
`compression` & `early_cpio` metadata.
The initrd.py can also use as a script that will print the dict
from `read_initrd` as JSON.
The implementation is a heavily based on dracut's `lsinitrd` bash
script and additionally contains a python port of the skipcpio.c
utility.
