Added some robustness to the WordShuffleModel, Passage, and ParsedText
classes to handle input that could cause problems (e.g. a string of all
the same words). I also added formatting features to the TextUnit class.
Now words can be displayed in various forms (first character only, first
and last character only, random characters displayed or no characters
displayed). That combined with the different phrase selection orders
(sequential, random and reverse) offers several variations on the same
exercise.