Append article to `trend_terms()` defaults

[rjake]

Use different defaults

trend_terms <- function(more = "an increase",
                        less = "a decrease",
                        same = "no difference") {

  ...

}

instead of https://github.com/rjake/headliner/blob/c6f29e9a220fb7a57f77972c1a89124b0ddacb36/R/trend-terms.R#L11-L13