HACK: compute total time on the review queue

[grunweg]

Note: carefully check if this is the right version of the branch before merging. Very possibly, #44 should be merged instead!

Depends on #25 (whose code is ready and just needs further clean-up). This PR adds a hacky semi-automatic step to scrape the information from a PR; see HOWTO_scrape.md for details.

To land this code, one should

• merge #25 first (after the necessary clean-up there)
• decide if this hacky scraping approach is sustainable enough to actually merge this
• code clean-up: a few things coming to mind are
  • reduce duplication in the "parsing label change lines" code (this is mostly the same logic, repeated with variations five times)
  • move the replacing of "awaiting-review" with "awaiting-review-DONT-USE" to a more central place

[grunweg]

If you manage to extract this data from the github API, I'd be even happier. I didn't find any way... but it'd be nice if you could prove me wrong!