Provide syntax for better CSS & JS asset management in custom theme .info files

This is derived from discussion at the BADCamp 2012 Twig sprint: http://groups.drupal.org/node/265858 Please see these notes for the background.

We'd to provide a mechanism in a custom theme to disable the CSS files and JS files added by contrib modules (some sort of simple asset management). This makes overriding contrib styling easier: instead of copying an entire CSS file and then changing all the properties, we can simply exclude the file from being included in the first place and write our own CSS.

For example, a custom theme could implement the following syntax in the .info file. These are examples in order from most granular to least granular (Arguably, the latter examples may be overkill).

  ; Use case i.
  ; Use sub elements to exclude individual CSS or JS files.
  assets[fivestar][base.theme.css] = FALSE
  assets[fivestar][admin.theme.css] = FALSE
  assets[fivestar][fivestar.js] = FALSE

  ; Use case ii.
  ; Exclude all CSS or JS files from a contrib.
  assets[fivestar][css] = FALSE
  assets[fivestar][js] = FALSE

  ; Use case iii.
  ; Exclude all assets (both CSS and JS files) from a contrib.
  assets[fivestar] = FALSE

  ; Use case iv.
  ; By default, we'll have a whitelist policy: all assets will be included
  ; and we'll want to exclude them on a case-by-case basis. However, we
  ; could change the default so that all assets are *excluded* by default,
  ; and we can include them on a case-by-case basis.
  ; Disable all contrib CSS files by default and perhaps simply enable fivestar's CSS
  assets[contrib][css] = FALSE
  assets[fivestar][css] = TRUE

The syntax above is just an idea, so we'd like to clarify this. This is not just limited to contrib modules, but could also work on core assets (if the themer dares).

@todo Use issue summary template :)