Add update flag so that unchanged files are not compiled.

This PR is to add an option for --update (update: true), when passed as true we use the SASS native `sass --update source:dest` and will only update if the file is recently changes.

This is great for compiling lots of individual stylesheets, or when a project scaffold contains many different folders so a generous globbing pattern is needed.
```js
var sassPath = args.shift() + ':' + args.shift();
```