PHP-1028: Add $options support to ->aggregate()

Add support for additional $options array when using a single array of pipelines

$pipeline = array('$operator' => stuff);
Previously the proto for this function was:
->aggregate($pipeline [, $pipeline [, $pipeline]])
or
->aggregate(array($pipeline, $pipeline, $pipeline))

Now we support also:
->aggregate(array($pipeline, $pipeline, $pipeline), array("option" => "value"));
e.g., there is a new optional argument when using the array($pipeline1, $pipeline2) mode.
This is consistent with how the shell does this.

This makes it possible to pass arbitrary arguments to ->aggregate(), such as:
- array("explain" => true)
- array("maxTimeMS" => 100)