Addresses #304.

Pre-Merge checklist

• All tests on CircleCI have passed.
• All visual regression differences have been approved.
• If changes are significant, a release candidate has been created and posted to Slack, the Plotly forums, and at the very top of the pull request.
• You have updated the dash/version.py file and the top of CHANGELOG.md. For larger additions, your CHANGELOG.md entry includes sample code about how the feature works. The entry should also link to the original pull request(s).
• Two people have 💃'd the pull request. You can be one of these people if you are a Dash core contributor.

Post-Merge checklist

• You have tagged the release using git tag v<version_number> (for the contributor merging the PR).
• You have pushed this tag using git push <tag_name> (for the contributor merging the PR).
• You have deleted the branch.

Pre-Release checklist

• Everything in the Pre-Merge checklist is completed. (Except the last two if this is a release candidate).
• git remote show origin shows you are in the correct repository.
• git branch shows that you are on the expected branch.
• git status shows that there are no unexpected changes.
• dash/version.py is at the correct version.

Release Step

• python setup.py sdist to build.
• twine upload dist/<the_version_you_just_built> to upload to PyPi.

Post-Release checklist

• You have closed all issues that this pull request solves, and commented the new version number users should install.
• If significant enough, you have created an issue about documenting the new feature or change and you have added it to the [Documentation] project.
• You have created a pull request in [Dash Docs] with the new release of your feature by editing that project's requirements.txt file and you have assigned @chriddyp to review.