* master: (30 commits)
  Elucidate & implement consistent principles for discrete NA
  Change meaning of trim (tidyverse#1802)
  Document strip.placement theme setting
  Style tweaks for theme()
  More sophisticated facet var evaluation strategy
  Teach rd_aesthetics about optional aesthetics
  Allow defintion of optional aes
  Link to geom_count from geom_point help
  Improve geom_density docs.
  Support hms position scales
  Correct aesthetic specs for bar charts
  Add check.aes and check.param args to layer()
  Handle sign transforming transformations
  Axis labels should always be character
  Only format numbers
  Update revdep email
  Re-run revdeps
  Run revdep check
  fix stacking (tidyverse#1817)
  Fix axis names for FacetNull (tidyverse#1814)
  ...