Add support for programmatically reordering objects

[joshuahiggins]

This idea stems from 2 use cases:

• Angular renders static markup faster than dynamic markup, such as markup generated via ng-repeat. When generating static and dynamic objects, Packery will append the dynamic after the static despite their original source order.
• A need to maintain user preferred order and/or positions of Packery objects. For instance, in a dashboard view with multiple widgets, a user may re-order widgets and wish to maintain this preference through later sessions.

Adding support to manage object order and/or positions could potentially address both issues. However, as Packery supports any grid size (down to 1x1 pixel), managing order would also require managing positions.

I'm not sure if this is something that should be added on a project basis, to fit individual needs, or via this modules where I believe it could potentially introduce a larger amount of fringe case bugs.

I'm opening this issue to come back to at a later time and hopefully get some feedback from others.