Avoid having to write on disk the image

[ChrisCinelli]

I worked on fixing this during another PR but it looks like that, even eliminating concurrency, the tar.Reader does not behave as I wanted and ended up cutting some bytes.
On the other side when it was working, the initialization was super fast.

[ChrisCinelli]

I was actually able to make it work. The code is on https://github.com/ChrisCinelli/dive/tree/feature/cc/do-not-write-image
Concurrency is off but dive loads a lot faster compare to the usual time on my machine and memory consumption is always around 16Mb =)

[wagoodman]

Fixed by #108 , thanks for digging in on this!