Refactor modeling, parsing, and client codes

Changes

separated modeling and parsing codes
- renamed client::parser to client::model
- added parser module
- moved parsing codes from client to a dedicated parser module
split client data models (client::model) and HN data models (new model module)
split parsing codes into different modules
- html for parsing HTML text
- article for parsing web article
and many other cleanups, variable/function naming changes, code refactoring, and more documentation