Add configurable limit to merge train sizes

[rudymatela]

This is a follow-up from #77.

#137 introduced merge trains. These are currently unlimited so merge trains can grow indefinitely. It may be interesting to have a field in config.json to provide a maximum number of PRs in a train.

This is not a huge problem, over the course of a month before merge trains, a busy project 😉 would be at most 9, but most of the time under 4. In the same situation now maybe not even 9 would be reached as we expect things to be merged faster.