Implement a new preprocessor that allows just words similar to those
at CSCW-2.txt. For each word within that file, it add synonyms derived
from WordNet. The name of this preprocessor ir AlternativeTextPipelinePreprocessor.

The previous preprocessor is available as DefaultTextPipelinePreprocessor. You
may use it instead (you just have to instance it instead of the alternative one).
The default preprocessor also employs a NLP strategy to remove even more words.

As of DTM, we can define more parameters now: minIterations, maxIterations, and
alpha.