support merge keyword in commits in order close PRs as merged from console.

[gaboesquivel]

I use hub to manage pull requests and I would like to be able to mark PR as merged using the merged keyword in the commit message.

git am -3  repo/pull/number
git commit --amend 
or
git rebase --options
## add merged keyword
git push origin master

I currently use merge and close #number for reference but that doesn't mark the pull request as merged