Extend _.groupBy to support multi-key objects

The current groupBy function only adds each Collection object to the returned hash only once.
In some case one might want to add the same object in multiple groups.

A concrete example is an object having an array property.

Basically this means replacing:

.groupBy = group(function(result, value, key) {
if (.has(result, key)) result[key].push(value); else result[key] = [value];
});

by

function addToGroup(result, value, key) {
if _.isArray(key) { .each(key, function(k) { addToGroup(result, value, k); }); return; }
if (.has(result, key)) result[key].push(value); else result[key] = [value];
});

_.groupBy = group(addToGroup);

I implemented this for my backbone collection as groupByMulti but that is rather cumbersome as I don't have access to the underscore internals, in particular the group function.

Start with _.groupBy and the internal group helper described in the issue, then review the existing collection utility behavior. Verify that a collection object returned with multiple keys is included in every corresponding group while single-key behavior remains unchanged; the payload does not name a test file or command.