allow main array for packages

[guybedford]

Consider bootstrap for example, it should be possible to load the js file without getting all the css.

Thus if we could define:

package.json:

{
  "main": ["js/bootstrap", "css/bootrap.css!"]
}

Then we would have:

  jspm.import('bootstrap'); // loads css + js
  jspm.import('bootstrap/js/bootstrap'); // loads just the js

[guybedford]

This doesn't make sense for a few reasons:

1. We no longer have simple pointer modules, but would need AMD exceptions. Or we have to have a new dummy main which is created.
2. It is not clear which item in the array is the exported value itself. Perhaps the first one could be the convention.

But I think this should have some stronger use cases before actioning.