src: Add a new Dictionary Class

[pangiann]

Dictionary class builds a dictionary of words loaded from various descriptions found in millions of books from the Open Library. To specify the book from which this class will get the description a Book ID is used.

In a nuthsell, this class does the following:

1. Checks if dictionary of same type/ID already exists.
2. If yes:
   • Finds the path to the file
   • Loads the dictionary from the file
   • Validates the dictionary
   • Returns the newly created object for later use
3. If not:
   • Gets the book's description using the ID
   • Processes description to create a valid dictionary
   • Saves it to a file
   • Returns the dictionary

Refs: https://github.com/pangiann/hangman/issues/1