Tokenize a text file and store into a dictionary

A text file (hardcoded) is tokenized, stripped from withespaces and newlines.
Trigrams (sets of three consecutive tokens) are stored in a dictionary, the
first two tokens as key and the last as value.
Prints 200 words of auto generated text

Starting from a random tuple, a third word is extracted and printed. The word
contributes then to the tuple (i.e. the tuple is shifted) and the process is
repeated 200 times.
Generate 200 words text -- code cleaned

The code is similar to the previous commit, but a bit cleaned