PRs script updates

[zmoon]

Fixes #283

• removed attempted optimization that was causing merged PRs to be missed when a previously merged PR had another "update" after being merged
  • unfortunately, it doesn't seem to currently be possible to sort by merge date in the API request
• for now, the whole list of merged PRs is searched, which takes a few minutes
  • with progress bars if tqdm installed
• PR titles on new line in the generated Markdown
• sort merged PRs by merge date (earliest merged -> first in the output list)

[LKedward]

Thanks for the quick fix @zmoon - changes look good to me and it works nicely as expected 👍

[milancurcic]

Thanks, @zmoon!