#27 inline critical css

Short description to the made changes:
* Take HTML and generate critical-path CSS
* Write criticalPath CSS to file if `opts. styleTarget` is set
* Inject the generated CSS String inline into the HTML
* Wrap existing link tags inside noscript tag for browsers with Javascript disabled
* Add script before the closing body tag to load stylesheets asynchronously 
* Write HTML file if `opts.htmlTarget` is set
* Return generated html as result in callback