pass grunt srcFile destFile and options to stylus renderer

In my case, I want my stylus library could be imported by others.
with the powerful `paths`, `use` and `import`, most things works fine.

But when we involve image, and wishes to support ie6/7 which means use stylus.url() to embed image is not acceptable.

In my approach, I write my own urlfunc called urlcopy to take relative images to the dest css path.
So I need the grunt task to tell stylus, where the destFile is through option of stylus()