Ability to load config module through data-config attribute

When using requirejs with multiple pages, it is often helpful to place the requirejs.config paths and shims in a separate module in order to be share amongst all the pages.

This is what I have been doing so far:
```html
<script src="require.js"></script>

<script>
  require(["config"], function() {
    require(["main"], function() {
    })
  });
</script>
```

After doing it this way for a while, I thought it would be cleaner to do it this way instead:
```html
<script data-config="config-module" data-main="main-module" src="require.js"></script>
```

I didn't spend much time on this yet, I basically duplicated the section in the code that handles the data-main attribute and renamed the attributes to data-config.