Version 5.3 (#14)

Backwards Compatible Interface Changes

• Sponge plugin arguments: pdml2flow +json '-0 -h' -> pdml2flow +json -0 -h
• Add Flow().id property

Improvements

• Remove plugin submodules
• Fix deprecated usage of resource_filename
• Test for python version: 3.5-dev 3.6 3.6-dev '3.7-dev