(tetris): New defgroup.

(tetris): New defgroup.
(tetris-use-glyphs, tetris-use-color, tetris-draw-border-with-glyphs)
(tetris-default-tick-period): Convert to defcustom.
Added * in docstring to indicate user variable.
(tetris-update-speed-function, tetris-mode-hook, tetris-tty-colors)
(tetris-x-colors, tetris-buffer-name, tetris-buffer-width)
(tetris-buffer-height, tetris-width, tetris-height)
(tetris-top-left-x, tetris-top-left-y): Convert to defcustom.