Adds check for transform function in modularizer

I stumbled upon an error due to a `.DS_Store` file existing in the tests folder:
```
TypeError: undefined is not a function
    at /Users/storgaard/Sites/billysbilling/modules/billy-webapp/node_modules/billy-builder/js-modularizer/js-modularizer.js:44:24
    at fs.js:266:14
    at Object.oncomplete (fs.js:107:15)
```

This PR adds a check to ensure a transform function exists for a specific file extension before trying to run that transform function. If it doesn't exist the file in question is skipped.

/cc @sebastianseilund