Minor improvements to Rd lexer.
Minor improvements to Rd lexer.

tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments,tomstuart/pygments