Assistance on pull requests

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`, ....