Add support for CASE expressions when tidyverse = TRUE

[ianmcook]

Add support for translating SQL CASE expressions to R expressions that use dplyr::case_when() in the tidyverse = TRUE case. This should support both forms of the SQL CASE expression:

CASE a
  WHEN b THEN c
  WHEN d THEN e
  ELSE f
END

CASE
  WHEN a == b THEN c
  WHEN a == d THEN e
  ELSE f
END

[ianmcook]

Added in a51fc01