Some great features

Asset Rack is incredible. Really.
I have always been saying "I should write some asset manager to
do name cache busting", but I never found the time.
This library does this and more.

But I found that some features are missing for it to be really usable:
- [ ] Implement `relativeUrl` (the base for the next feature),
  which returns the URL of an asset relative to another asset.
- [ ] **Have a way to reference an asset** (i.e. an image) **from inside
  another asset** (like a Stylus stylesheet).
- [ ] [RequireJS](http://requirejs.org) is a module loader. It helps you
  organize your scripts and define dependencies between them.
  **And it has an optimizer** which concatenates scripts in the correct
  order, optimizes and optionally minimizes them.
  Asset-Rack should have some support for it.

And some other gotchas:
- [ ] Be flexible. Don't *require* some things to be present, make them optional.
- [ ] Allow for more customization for the Stylus asset. That includes:
  * Ability to not use NIB. NIB is excellent, I always use it
    but there may be some people who prefers not to.
  * Custom configuration of the Stylus compiler used.

  To not break compatibility at all, I propose to add an option
  named `config`. It should be a function so we can `stylus.use(...)` it.
  **If not set, `nib()` is used instead** (if available).

Also, why does `compress` default to `false`?
Come on, the static web is efficient. :rocket: