Use config-extend instead of node.extend

node.extend merges arrays, which is virtually never what we want for
merging config options:

```coffeescript
defaults:
  key: [ "1", "2", "3" ]
override:
  key: [ "a", "b" ]

extend(true, {}, defaults, override) #=>
```

What we want is recursive merging but arrays are overwritten, not
merged:

```coffeescript
defaults:
  key: [ "1", "2", "3" ]
override:
  key: [ "a", "b" ]

extend(true, {}, defaults, override) #=>
```

The first option is optional, just like every other extend library.
However, in this library, it defaults to true if omitted. I'm leaving it
in for clarity. (and in case we have to change extend libs again)
Merge pull request #103 from testdouble/config-extend

Use config-extend instead of node.extend