SourceMap and Caching

This release includes improvements by @neekey and @viveksjain targeting sourcemap improvements and caching.

With the improvements from @neekey, the sourcemap option for the compiler should be better at returning the correct line numbers in the css map.

Working with large numbers of less files also became easier thanks to @viveksjain who implemented a cacheFile for saving "the imports (in JSON format) to the given cacheFile on exit." This can greatly improve startup time when nothing has changed in the fines.