New nicer version of the catchup script

[BlueCutOfficial]

Relates to #220 (doesn't close it because I consider replacing the argument parser)

This PR implements a new, nicer version of the catchup script:

• The script is split into several files for better readability and maintenance.
• Multiline messages are returned by dedicated functions to avoid big text blocks in the middle of the logic, making the process easier to follow.
• Additionally, requests to the GitHub API are managed by an instance of a GithubAdapter class that can be constructed with all the parameters required for the different requests.
• The possible execution paths have been simplified: the script doesn't make it to the API requests if there's an issue with file writing and never updates ref-upstream nor switches back to master if there's anything manual to do.
• The library chalk is used to add colors and styles to the different messages, which greatly improves the readability of the output.
• shelljs was used in the 1st version of the script, it should have been uninstalled sooner.

Note it has been implemented and tested in a dedicated repository.

[BlueCutOfficial]

I want to run the script soon enough, so I'll merge this since it has been open for a while and it's a bit hard to review. There are still improvements to make afterward.