Allow keepSpecialComments to be set as a string

On grunt-contrib-cssmin, some users are setting `keepSpecialComments` as a string instead of a number. This causes `undefined` to be prepended to their file. See gruntjs/grunt-contrib-cssmin#40

This would make setting that option with `'1'` or `'0'` work the same as a number directly. Hopefully this will prevent the error from their common mistake.

Thanks!