Option to generate LESS sourcemaps.

Since versions 1.5.0 LESS supports generation of sourcemaps. The patch adds an option to use the with the extension.

Sourcemaps allow developer tools of modern browsers to map the output CSS back to the source LESS which makes it possible to show the correct line numbers.

To make auto-reload after editing work in Chrome we would have to automatically trigger a compile when the source LESS file changes. This isn't implemented yet.