kbenoit/quanteda: CRAN v0.9.9-17

Bug fixes and minor feature additions.

• Fixed a bug causing dfm and tokens to break on > 10,000 documents. (#438)
• Fixed a bug in tokens(x, what = "character", removeSeparators = TRUE) that returned an empty string.
• Fixed a bug in corpus.VCorpus if the VCorpus contains a single document. (#445)
• Fixed a bug in dfm_compress in which the function failed on documents that contained zero feature counts. (#467)
• Fixed a bug in textmodel_NB that caused the class priors Pc to be refactored alphabetically instead of in the order of assignment (#471), also affecting predicted classes (#476).

• New textstat function textstat_keyness() discovers words that occur at differential rates between partitions of a dfm (using chi-squared, Fisher's exact test, and the G^2 likelihood ratio test to measure the strength of associations).
• Added 2017-Trump to the inaugural corpus datasets (data_corpus_inaugual and data_char_inaugural).
• Improved the groups argument in texts() (and in dfm() that uses this function), which will now coerce to a factor rather than requiring one.
• Added a dfm constructor from dfm objects, with the option of collapsing by groups.
• Added new arguments to sequences(): ordered and max_length, the latter to prevent memory leaks from extremely long sequences.
• dictionary() now accepts YAML as an input file format.
• dfm_lookup and tokens_lookup now accept a levels argument to determine which level of a hierarchical dictionary should be applied.
• Added min_nchar and max_nchar arguments to dfm_select.
• dictionary() can now be called on the argument of a list() without explicitly wrapping it in list().
• fcm now works directly on a dfm object when context = "documents".