v0.2.0

Add Feature: Find task by keyword

Dongji does not support find task by keyword.

When task list gets exhaustive and the user cannot see all tasks at once, find by keyword is necessary, but Dongji doesn't know how to do it.

Add Find task by keyword, by creating FindCommand and implementing finding logics within. Add 'find' command parsing to CommandParser so that it can understand find command.

A new component FindCommand is necessary to represent new type of Command. Then, update Parser so that it can  identify "find" from the user input.

Let's add FindCommand and modify CommandParser to enable 'find by keyword' feature