Add the Lexicoder Sentiment Dictionary

[kbenoit]

Create a data_dictionary_LSD2015 object (already done in the textscaling package) and be sire to include the negations in LSD2015_NEG.lc3.

Need to pay attention to the licensing disclaimer about non-commercial use, and get approval from Soroka and Young before publishing. (Preliminary approval already given by email.)

[stefan-mueller]

I was wondering whether we should print a message in the console to cite Young and Soroka (if it's used in a publication) when data_dictionary_LSD2015 is part of a command? I think it's a great service by the authors to make their dictionary available in quanteda, and they deserve the acknowledgement.

Also, we might update the manual and say that [neg_negative] and [neg_positive] can only be applied to a dfmwith bigrams (if that's correct)?