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
 - [ ] add documentation / docstrings
 - [ ] figure out how to display added/removed spaces



we've worked hard to minimize the amount of code in main.py, so this whole block should definitely go in a dedicated function named `review_pr` that is simply called from main; it should probably live in `github.py`