Less source map option

`lessc` require file name for `--source-map` option since this script used the > character to output the css file. Full command look like this:
```
lessc style.less > ../css/style.css --source-map=../css/style.css.map
```

There are some workaround to add the option directory to `lessc_cmd` but it will be simpler if this plugin have an option to toggle on / off this flag.