add switch-base and squash pull request when merging
include all the authors in the merge title

This uses the pull-request commits as we need the user names
and not the author-names. And also makes it possible to keep
credits when squashing commits
clear temp branches on command end
fix wrong placeholder for authors
Merge pull request #361 from sstok/199-extra-options-for-merging-pull-request

add switch-base and squash pull request when merging