Use chokidar for file watching; read already0compiled HTML files directl...

This branch uses [chokidar](https://github.com/paulmillr/chokidar) as a replacment for Node's `fs.watch`, which should yield more consistent file change tracking across platforms. With `fs.watch`, only the first change to a watched file was tracked (when editing with Sublime Text on OSX 10.9). This problem is resolved when using chokidar.

I've also modified the code so that the `command` argument is only run when the tracked file has a ".md" extension (this should probably be extended to other common markdown extensions). This allows me to watch an HTML file that has been generated by another Markdown processor, such as the [knitr](http://yihui.name/knitr/) extension for R.