Allow to specify default PR merge option

[MGChecker]

Currently, there are four different ways to merge PRs

• Merge pull request
• Rebase and merge
• Rebase and merge (--no-ff)
• Squash and merge

Usually, the first one in the list which has been activated via repository settings is the default and is achieved when clicking on the big button. I would like to be able to specify the default action for the large button in the repository settings.

[lafriks]

Usually only one per repo should be used so imho best practice would be to disable all other options in repository settings