Add --set-commit-status flag to merge command

Setting this flag with create a success or failure status on the
latest commit of all PRs in the merge, depending if all PRs could
be merged or not.
Add new set-commit-status command

Based on merge, this uses the same filters etc. to find all relevant PRs
and then sets the given commit status on those PRs.
merge master -D org -I include -E exclude: PR 82 (Add logic to create tag while updating submodules)
merge master -D org -I include -E exclude: PR 85 (Test fix)
merge master -D org -I include -E exclude: PR 86 (Add support for Github commit status)