RequireJs Almond replacement

This transform replaces requirejs with almond.

Syntax:
``` html
<script data-almond="path/o/almond" src="path/to/require.js"></script>
```

It checks if requirejs is used for external script loading and emits a warning if that is the case.

Closes https://github.com/One-com/assetgraph-builder/issues/52