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



use `download_repo('easybuild-easyconfigs', branch=review_pr_to_branch, account=review_pr_to_account)`, and look into additional options for controlling the branch/account to which the PR is compared