Feature linter formatter

[simwai]

I extended the functionality of next lint and added stylelint to lint css. The stylelint rules are mostly a merge of the stylelint-config-standard and stylelint-config-recommended rules. I know every coder has its own style, so feel free to adjust some rules, if you don't like them. Moreover, I updated the README to use a venv to avoid dependency conflicts (which I got without it) and added source_documents to the .gitignore file. Additionally, I cleaned up the requirements.txt file using pipreqs.

[simwai]

@joemccann @Anil-matcha @staeiou Heyo, would be appreciated if anybody can have a look on this.