Added preprocessor for docs references as paths in CSV file
Added preprocessor for docs references as paths in CSV file

Defaults and Customization:

     * The input file is treated as a simple text file having one path per file. However, if the input is a CSV file,
       a pair of ``column`` and ``delim`` parameters may be used to retrieve the desired value as reference path.

     * The referenced documents are treated as text document and hence parsed using ``TextDocPreprocessor``.
       However, if the referenced files are complex, an advanced parser may be used by specifying ``parser_factory``
       parameter to constructor.

HazyResearch/snorkel,jasontlam/snorkel,jasontlam/snorkel,HazyResearch/snorkel,jasontlam/snorkel,HazyResearch/snorkel