Make stripping punctuations configurable option.

[stultus]

There are cases where the normalized text is expected to retain the punctuations. As of now, the normalizer removes all punctuations. It will be good to make this a configurable option by passing an argument to the normalize method.

[asdofindia]

Have added a keep_punctuations option to normalize(). This should now be possible with:

normalize("some text", keep_punctuations=True)