Enable gulp-sourcemaps to work automatically

This might not be a road you want to go down, but I rewrote the plugin to use Stylus directly, completely removing Accord from the equation.  I ported over Accord's option processing (although much simplified from the generated CofeeScript), so all the original functionality exists without any changes.

Existing code should not be affected in any way.

The benefit is that simply activating sourcemaps as directed by `gulp-sourcemaps` automatically enables sourcemap processing.  This change *also* allows developers to apply changes prior to `gulp-stylus`, so you can do things like concat files before stylus processes them, and get the original sourcemap.

To make this change, I:

* Rewrote the existing code
* Removed the Accord library
* Added the vinyl-sourcemaps-apply library
* Added a test for the new functionality
* Updated the README to reflect the recommended `gulp-sourcemaps` technique.
* I also did my best to match your existing code style, although I might have changed some things accidentally because there was no editorconfig, and I think some files had tabs, and other had spaces.

If this is something you would like to use, let me know if you need me to make any changes.