Check package.json for 'less' property

In order to allow for packages that have both less and css in them (like twitter-bootstrap), I thought it would be a good idea to default to checking a `less` property on package.json before checking for a style property.

This would allow module authors to distribute both a compiled css file and a less file, and have it work with `npm-css`, `rework-npm`, and `npm-less`.