build.corpus: Build a corpus that can be used in the textreg call.
Description
Pre-building a corpus allows for calling multiple textregs without doing a lot
of initial data processing (e.g., if you want to explore different ban lists or
regularization parameters)
A list of strings or a corpus from the tm package.
labeling
A vector of +1/-1 or TRUE/FALSE indicating which documents are considered relevant
and which are baseline. The +1/-1 can contain 0 whcih means drop the document.
banned
List of words that should be dropped from consideration.