Assistance on pull requests

[cebe]

Github allows maintainers to push to pull request branches if the author allows it. @yii-bot is a maintainer too, so we could automate things like

• adding a changelog line
• running code style fixes
• ...

e.g. by adding a comment: @yii-bot add changelog: Enh #123: Something was added (author)

@yii-bot apply csfix, ....