Fork API documentation

[scripthunter7]

Preview

TODO list:

• Basic example, introduction
• Add link to https://github.com/csstree/csstree#documentation
• Add link to https://github.com/csstree/csstree/tree/master/docs#readme
• Cover the following options:
  • generic
  • units
  • types
  • properties
  • atrules
  • node
  • pseudo
  • scope
  • features
  • atrule
  • parseContext
  • tokenize
• Mention fork(callback) case

[coveralls]

coverage: 98.866%. remained the same
when pulling d9e87f4 on scripthunter7:feature/fork-api-docs
into ba6dfd8 on csstree:master.

[scripthunter7]

@lahmatiy I made a basic version over the weekend, there are still some TODOs, so the PR is only a draft for now. If you have time, check it out please, I'm interested in what you think) I hope I didn't forget anything, I tried to collect the most important things.