Add CLI option to analyse *open* PR statuses

[jayvdb]

For each repository, check the status of open PRs.

To make this useful, the date of the last master green build is needed. And if the latest master build is red, analysis of PR status can be omitted unless given a second CLI argument to --force the check.

It is not especially interesting to know that a recent PR is red, if there was a newer PR which is green. It means the red PR is buggy.

It is also not especially interesting to know that the most recent PR is red, if there was a green PR recently. It means that the green PR possibly needs to be merged to fix master, and then all red PRs need to be rebased.

Some suggested analysis

• How many PRs are open
• Date of most recently created PR which is green
• Date of most recently created PR which is red, if more recent that all green PRs, and the build was greater than a certain number of days
• How many PRs are green CI? (maybe list the most recent few by PR number only)

[jayvdb]

This issue reserved for Google Code-in task 6539192793300992.