Add support for dockerfile stages

[MartinBasti]

Currently dockerfile-parse focus only to work with the latest stage.

Add support for doing modifications and reading content per dockerfile stage.

See https://github.com/containerbuildsystem/dockerfile-parse/issues/116#issuecomment-794155530 for context

[ElijahAhianyo]

HI, I'd love to take this up

[MartinBasti]

Nice, thank you for willingness :)

IMO this can be quite a big change, we may need a design for this before coding.