Merge branch 'fdroid-yml-builds' into 'master'

builds straight from source repo using .fdroid.yml

The overarching theme of the merge request is allowing _.fdroid.yml_ to be included in an app's source repo, then letting `fdroid build` build the app straight out of the git repo without requiring a setup like _fdroiddata_ (e.g. _config.py_, _metadata/packagename.txt_, etc.).  _fdroiddata_ repos can then include source repos with a _.fdroid.yml_ by having _metadata/packagename.txt_ that includes just:

```
Repo Type:git
Repo:https://gitlab.com/upstream/app.git
```

Any other metadata fields that are included in _metadata/packagename.txt_ will override what is in _.fdroid.yml_, giving the repo manager the final say about what is included in their repo.  This setup provides a number of benefits:

* CI systems like jenkins, travis, gitlab-ci can build from _.fdroid.yml_
* very easy to start building apps using `fdroid build`, no separate repo needed
* some maintenance can be offloaded to the upstream dev

See merge request !184