Use the colors of the current theme in RStudio.

Color matching brackets to help distinguish between scopes
(#8, @mdequeljoe).

The withr package is now a development dependency, it is not needed
for regular use.

Add prettycode::prettycode(). Call this function to turn on
pretty-printing of function objects. This is needed to work around
the new S3 method search limits in R 3.5.x.

If prettycode fails to highlight a function, it falls back to
base::print.function() now (#3).