Sanitize json keys

Hi there,

I've got a json file that's essentially like:
```
{ "some key" : { "key": "value"} }
```

If I try importing this, sass loader throws `SassError: expected ":"` because the output scss is like
```
$some key: ( key: value )
```

I can change the key to `some_key` and it works fine, so there's a workaround. It would be nice if it would sanitize keys, though.