Add initramfs inspection commands

[anatol]

It would be useful to add support for initramfs inspection to booster command. It can look like booster ls $booster.img and booster cat $booster.img:/somefile.

For consistency we should add a sub-command for generatin an image booster gen that is going to be an equivalent for current booster invocation without any subcommands.

[anatol]

Candidates for commands would be: generate (or maybe gen the default command) ls unpack cat