Add content option to tabular pipeline

[davidmezzetti]

Currently, the tabular pipeline will create a pandas DataFrame for input data and create (id, text, tags) tuples. A new option should be added to also create additional content records.

The content field can be either True/False or a list.

• If False (default), no content is created
• If True, all fields available in the DataFrame are added to a content record for each row id created
• If content is a list, the list is used to select only those fields from the DataFrame to add to a content record