FeatureUnion example using a heterogeneous datasource.

Uses the 20newsgroups dataset. Extracts & processes the subject-line and body
with separate pipelines and combines them with a FeatureUnion.

Closes #3569
This may be true to some extent, but you could also state that we may expect a model where body and subject terms are weighted differently to outperform one where they are treated as a single text.