Adds GNOME Checker

[A6GibKm]

Adds a checker for gnome, see #91. Format should be

                    "x-checker-data": {
                        "type": "gnome",
                        "name": "baobab"
                    }

TODO:

• Update README
• Unit test for unstable package
• Unit test for package already using latest
• Use an example for the test which won't be updated, unlike libhandy
• Skip unstable releases, e.g. 3.37.x and 40.alpha
• Set skip-unstable by default
• Fix CI See Add JSON checker #90

[A6GibKm]

I don't know of an example of a package for which its latests version is an unstable one and won't be updated any time soon.

[gasinvein]

Superseded by #141