Markdown-full-yaml-metadata

Just update Readme and remove unneeded manifest file

- drop support for python 3.6 and 3.7
- use Poetry to manage dependencies
- change code style

Support for custom yaml loaders

Update dependencies and few minor improvements

New release:

* do not require testing dependencies in setup.py
* store only development deps in requirements.txt
* development\contributing guide for README.md

Require PyYAML~=5.0 and load yaml content in more safe manner