Support for pattern fills

[trevorld]

• patternFill() wraps patternGrob() to return a grid::pattern() fill object (#70).

• update_alpha() updates fill colour and/or pattern transparency.

  • It is a fork of ggplot2::fill_alpha() by @teunbrand.
  • It does not depend on {ggplot2} or {scales}.
  • It does not throw an error with a length one list of a vector of multiple colours.

• The "geometry" patterns (e.g. "circle", "stripe", etc.) now allow the fill to be pattern fills (#67).