Added underscore plugin

This plugin removes all symbols that begin with an underscore from doc output. This comes in handy when you're using underscores to indicate private variables that you want documented in code or internal docs, but not in published api documentation.
It should be safe to remove this line. JSDoc no longer adds the name `____` to object literals that have `@lends` tags.

If I'm right, please update the comment on line 16 as well.