Provide generated filename to associate with source map

This pull-request fixes [issue with sourcemaps](https://github.com/plus3network/gulp-less/issues/72) when using gulp-less with gulp-autoprefixer and other tools.
@floridoo you right, but i don't sure where it should be placed. If i will handle this case in gulp-less, then this kind of fixes should be applied to each gulp plugin which changes file name.

On the other hand this case should be handled only where file name have been changed.

So i really don't sure about that.