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



Please expand the above; I was compelled to ask Ken on what is this about, so, here are some ideas:
```
[10:39:11] <boegel> fotis2: --review-pr 123 # will compare all easyconfigs touched in PR #123 with current develop, and give an enhanced diff report
[10:39:59] <boegel> fotis2: for each easyconfigs, it will try and find a set of closely related easyconfigs (some software + e.g. same version + toolchain name), and show the differences in a summary view
```