Update README.md.
Update README.md.

* Add npm version badge
* Update usage info
* A few minor style tweaks

addyosmani/grunt-uncss,kingsj/grunt-uncss,simohaitham/grunt-uncss,simohaitham/grunt-uncss,tazminweb/grunt-uncss,sparanoid/grunt-uncss-inline,kingsj/grunt-uncss,tazminweb/grunt-uncss,zabefloyd/grunt-uncss,zabefloyd/grunt-uncss,addyosmani/grunt-uncss,sparanoid/grunt-uncss-inline