Initial document-independent feature extraction support

[lgrz]

For tracking a number of issues related to pre-retrieval document independent features.

• [ ] preret_csv assumes unigram and bigram files are text
• [ ] preret_csv assumes user must use both unigram and bigram files
• [ ] generate_term_features output is text
• [ ] create_bigram_inverted_index output is binary file
• [ ] create_bigram_inverted_index is missing ordered and unordered options
• [ ] add documentation for bigram features