feat(package): add important main file support

Allow plugin mains to be exported if specified.

Following #234 jspm/registry#62

---

Example :

```bash
jspm install select2 -o "{ main: 'select2.css\!' }"
```

Is now writing in `jspm_packages/github/ivaynberg/select2@x.y.z.js`

```js
module.exports = require("github:ivaynberg/select2@3.5.2/select2.css!");
```

So importing `select2` trigger the css plugin and import the css file.
Merge pull request #278 from douglasduteil/feat-package-support-important-main-file

feat(package): add important main file support