Idea: access parent cursor in doc helpers

That would be pretty useful to be able to access the parent cursor from a document returned to the helpers method, and in particuler its previous / next docs in the cursor.

That would make things like grouping elements by an attribute very clean. Examples: for a chat app where you would need to insert a separator for each day, a contacts list where you need to group people by the first letter of their names, etc.

The code could look like this:

``` javascript
Messages.helpers({
'$newDay': funtion(prev, next) {
return (!prev || (this.day !== prev.day)) ? this.day : null;
}
});
```

Haven't been able to dig into minimongo yet - but @dburles do you think this is something that can be done?