Feature/add review pr option

use `./eb --review-pr 1017 -D | less -R` to try this out.
TODO:
 - [ ] Compare with other files found in the same PR
 - [ ] add legend on top
 - [x] add documentation / docstrings
 - [x] figure out how to display added/removed spaces



I was bitten in the past by github tar.gz issues like the following:
https://github.com/hpcugent/easybuild-easyblocks/issues/313
What if the download extension is actually a tunable as a function initializer? (eg. allow for .zip) just an idea...