Add option ''modernizr' to 'Unused Classes'

I know, it could be easy reach by overriding rule, but i think that there are 2 issues about:
* Modernizr is popular and uses as basic tool to build modern web apps, so many developers would have to list classes on their own to avoid flood of "Unused Classes" warns
* Modernizr adds its classes to HTML tag, what isn't to simple override

Thank you and I'm sorry if something goes wrong - it's just my first pull request ;)