Renamed load_la to load_puz. Added 21x21 puzzle.
Can take a filename now
Adjusted font sizes.
Re-factored with Rubocop suggestion.
Refactored add_numbers_and_clues

The clues are now stored as one array of Clue (dir, num, and text)
and returned as across_clues and down_clues.
The clues are drawn.

Currently, they potentially fall off the end of the screen.
Clues that are too wide are wrapped.