Deprecate implicit text-only for script and style

closes #875

If we can remove this feature (probably in `0.32.0`) then it will simplify a lot of code and significantly reduce the amount you have to learn when you're new to node.

The first step to removing it is to deprecate it and discourage people from using it, so this pull request logs a warning whenever someone relies on this feature.  Dealing with the fallout from this will also help to gauge how many people we're going to annoy by removing this feature.

Crucially, once we actually remove the feature altogether, we will be able to write templates that do things like:

```jade
script
  import foo.js
```

which would be enormously useful.