Backwards Compatible Interface Changes
- [x] Sponge plugin arguments: `pdml2flow +json '-0 -h'` -> `pdml2flow +json -0 -h`
- [x] Add `Flow().id` property
Improvements
- [x] Remove plugin submodules
- [x] Fix deprecated usage of resource_filename
- [x] Test for python version: `3.5-dev` `3.6` `3.6-dev` `'3.7-dev`