Rewrite nlppipe as series of functions as opposed to class

[nateraw]

Class seems kind of unnecessary. Would be nice if it were all functions that you can call via a single function. i.e.

import nlppipe

texts = ["some document", "another doc"..."last document"]
nlppipe.process(texts)

On top of that, it would be helpful if this could just be called directly in run_20newsgroups.py, checking if preprocessing had already been completed (to the same specifications/parameters passed to process function)