.gitconfig: Add alias to merge GitHub pull request

* Usage: 

  ```bash
$ git mpr <number>
```

* Description: Running `git mpr <number>` will checkout the content from the pull request with the specified number into a branch called `pr/<number>`, and, if the changes can be merged into master without any conflicts, it will merge them on top of the `master` and amend the commit message by adding `Close #<number>` at the end of it.