Implemented: colors, error handling, status check.

Colors are executed by way of the colorama python module
(see readme) and standard ANSI sequences.

Error handling is now able to catch nonvalid user input
during gameplay; in particular, during mode selection
and during card selection.

New feature: status checker - now able to see how many
cards are left in the deck.

FURTHER NOTES:
- Allow the game to automatically end when there are no
more valid sets to be made
- Allow multiplayer functionality (likely by adding a
step in gameflow to input which player is guessing a set)
- Tracker to count how many sets each player has acquired;
to be displayed within status checker and at end of game.