Add Scalastyle checker

Utilizes Scalastyle from http://www.scalastyle.org
I am not convinced that this is a reasonable default.  If the user is not aware of this setting, the syntax checker will mysteriously fail for them.

I'd rather like to use a special symbol, e.g. `unset`, and check for this in the `:predicate` of the checker, with a message that directs users to change this setting in order to use the checker.