Added two checkbox parameters, autoCloseFailed and autoMergeStable

...ing each project to determine this automatic behavior if wanted.

This implements a request for auto merging #65, and my own personal request similar, to auto close but gives the option to the project configuration.

https://github.com/timols/jenkins-gitlab-merge-request-builder-plugin/issues/65
Related to my comment on a potential `NullPointerException`, this should be a `boolean` rather than a `Boolean`. That way anyone who has not specified this value (such as those who have upgraded the plugin) automatically get the default value (false for a boolean).