prepare_news: Prepare news data
In maskedforreview/gtdnews: Online appendix for @@ paper

Prepare news data

prepare_news(
  d,
  doc_col,
  date_col,
  text_cols,
  docvars = NULL,
  first_n_words = NA,
  rm_time = T
)

`d`	A data.frame in which each row is a news article.
`doc_col`	The name of the column with unique article ids
`date_col`	The name of the column with the date, in a format that can be coerced to a Date object with as.Date()
`text_cols`	The name(s) of the column(s) with article text. e.g. c('headline','byline','body')
`docvars`	The columns to include as docvars (article meta such as headline, url)
`first_n_words`	Optionally, only include the first n words of the article text in the DTM
`rm_time`	If TRUE, remove time related terms, such as monday, januari and yesterday.