Added the opportunity to use different style files

Added the opportunity to use different style files (P.S. not only .css files).
For example you can use .less style files.
P.S. see lesscss a CSS pre-processor.

    <script src="skel.min.js">
        window._skel_config = {
            preset: 'standard',
            fileExt: '.less',
            linkRel: 'stylesheet/less',
            prefix: 'less/style',
            resetCSS: true,
            breakpoints: { '1000px': { grid: { gutters: 25 } } }
        };
    </script>