Add ::propertiesForSource

I need to get all the settings for a given source name in https://github.com/atom/atom/pull/3718. So this.
I was super worried about ordering in the case of this: 

```
a = {}
a.b = 1
a.c = 2
a.d = 3
a.e = 4
a.c = 5 # Will c come before d int he loop? Empirically, it does
```

, so I didnt use a hash.