add flat and flat-square badge styles

Shields.io has two additional styles: `flat` and `flat-square`. It would be awesome if drone supports them, too.

This PR adds them.
Usage: `/api/badge/:host/:owner/:name/status.svg?style=flat`.

I have no problem with adjusting the implementation - If you'd like to have it implemented differently. E.g. using a struct for  the `success`, `failure`, `started`, `error`, `none` map.
yes, we always want to return an image