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.
This is the complete snippet (its from the `GET /api/badge/:host/:owner/:name/cc.xml` route; its not the `status.svg` route).

```go
	repo, err := datastore.GetRepoName(ctx, host, owner, name)
	if err != nil {
		w.Write(badges["default"][model.StatusNone])
		return
	}
	commits, err := datastore.GetCommitList(ctx, repo)
	if err != nil || len(commits) == 0 {
		w.WriteHeader(http.StatusNotFound)
		return
	}
```

If the repo does not exist, it returns a badge. However, if there is no commit, it returns a `404`. And if it succeeded it returns a response in `CCTray XML` format. 
That is, there is an inconsistency between the badge and the `404` response and furthermore I think, the badge does not comply to the expected `CCTray XML` format.