Add support for hexo within OpenShift's DIY cartridge

Allows to `hexo deploy` stright to a DIY application in [OpenShift](https://www.openshift.com/).

To use, configure your hexo deployment with:

```yaml
...
deploy:
  type: openshift
  remote: origin
  branch: master
```

It will copy the generated `/public/` directory to the configured `root`, add and commit the changes, and push to upstream. The `openshift.js` plugin is greately inspired by the `heroku.js` plugin.