Returns subsets of a corpus that meet certain conditions, including direct
logical operations on docvars (document-level variables). corpus_subset
functions identically to subset.data.frame(), using non-standard
evaluation to evaluate conditions based on the docvars in the corpus.
# NOT RUN {summary(corpus_subset(data_corpus_inaugural, Year > 1980))
summary(corpus_subset(data_corpus_inaugural, Year > 1930 & President == "Roosevelt"))
# }