Use optionally a different separator for memorystore 

I want to be able to have a different separator for the memorystore (and thus for file). For instance when I store a config like this ```foo:bar:baz``` I want that to be stored as a whole not nested.

```javascript
nconf.file({file: 'config.json', separator: '||'});

nconf.set('foo:bar:baz', 123);
nconf.set('baz||bar', 456);

nconf.save()
````

Will save this:

```javascript
{
  "foo:bar:baz": 123,
  "baz": { "bar": 456 }
}
```