Add support for pagelet priorities

[brikis98]

One of the features in Facebooks BigPipe is to label pagelets with different priorities. For example, the
most important pagelets above the fold, such as the top three news feed items, get the highest priority, and are rendered before anything else, no matter what order they get sent down the wire. Other pagelets above the fold are in a second priority group, data below the fold is in a third priority group, and non-essential features, such as chat, are in the lowest priority group.