Refactor Parser and its related classes

- Add in new command, ADD for parser to take in first before it splits
to add_event, add_task, add_plan for engine
- clarify InputData documentation
- refactor out all magic numbers in Parser.java but have yet to arrange
them
Refactor Parser and ParserDate

- put all magic number constants at the top of Parser Class
- name all magic numbers in ParserDate
- need to choose one method of refactoring that makes sense
Created new classes

- created ParserAdd, ParserEdit and ParserList