Upload files locally

This adds the ability to upload images directly into the roots project that roots-cms is managing.

In the roots project `cms.json` config, the user can specify an `uploader` and `img_upload_dir` key.

`uploader` can take `fs` or `s3`. defaults to `fs`
`img_upload_dir` is the directory in `assets/img` roots-cms should save to. defaults to `uploads`.