Add fileExt & importOption options, ability to use global less

Hello,

I've made several changes to improve usability of the plugin. Namely:
* `fileExt` option to specify default file extension
* `importOption` option to set the way of resource importing
* ability to use preloaded global `less`
* ability to use global `less` configuration
* adaptation to `curl` configuration
* the example to demonstrate usage of new options