Merge: Beef up github merge

The tool was fragile, crappy, and very limited.
Now it is still fragile, crappy but less limited.

A lot of new features:

* can work on other repositories than nitlang/nit
* can choose what query to list (eg label)
* fixed status info, and we can filter on it also
* can merge automatically
* and ~~much~~ some more.

All these improvement are used to enable the automatic construction of a `next` branch based on PR with some characteristics.

Pull-Request: #2293
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>