WIP: --split-configs flag

I've been mailing with @addyosmani about investigating the possibility of adding a `--split-configs` flag, which allows you to generate the grunt config into several config files.

The most significant change is that I had to extract the entire grunt config into a separate file (`_config.js`) in order to be able to process it one way or the other. 

I wrote a test, but unfortunately I can't run the tests, I get:

```shell
Uncaught Error: You don't seem to have a generator with the name webapp:app installed.
```

Not just with my branch, but with the master as well. (I've tried re-installing generator-webapp, npm cache clean, etc. etc. All to no avail though)
I did however test it out pretty thoroughly manually and it works well. 

Let me know what you guys think!