Added additional challenge to 04-cond to combine conditional, per discussion with @jdblischak
replaced parameter with argument, identified argument with a defualt value of TRUE, and changed name of argument to use_boxplot
Updated new challenge wording to drop boolean and instead refer to the use_boxplot argument
Merge pull request #779 from jhollist/master

Added additional challenge to novice R lesson 04-cond to combine conditional statements.