Made cheerio selectors and mapping an option.

Allows users to add an option:

    filters: {
        $selector : function | function[]
    }

for example, to also include requirejs main.js scripts:

    filters : { 
        'script' : [
            function() { return this.attr('src'); },
            function() { return this.attr('data-main');} // for requirejs
        ]
    }