I think we should also consider passing preprocessors in a form of nodejs modules, saying I have a nodejs module that export a function, and I want to install that at the top level as `devDependencies`, then passing that name into the `preprocessor` option. For that, I think we can do:

```
try {
   preprocessorModule = require(preprocessor);
} catch (e) {
   preprocessorModule = require(path.resolve(preprocessorsRelativeTo, preprocessor));
}
```