Dockerfile-parse

Improvements
- Removal of python-six module

Compatibility changes
- Python 2 support has been dropped


Improvements
- support --platform parameter in image_from
- migrated tests to tox for easier testing and development

Improvements
- Avoid adding repos to from scratch with alias

Improvements
- Added support for the [`escape` directive][]

[`escape` directive]: https://docs.docker.com/engine/reference/builder/#escape

It was agreed by maintainers that maturity of this project is good enough to release major version 1 \o/ .

Improvements
- Added support for ARG statement handling

- minor code fixes