Squashed and fast forward merges
You can now squash your pull request before merging and edit the merge commit message, bringing Reviewable back to parity with the GitHub UI. And by way of apology for how long you had to wait for this I also threw in support for fast forward merges!
If the pull request's branch is out of date you can now update it by merging the target branch into it from the checks dropdown.
Finally, you can limit the kinds of merges allowed in your repos via the repository settings panel. (Unfortunately, GitHub doesn't expose their own equivalent settings via the API.)
