Add an async driver for document rendering and a crude "Hexdump" document engine

Summary: Depends on D19237. Ref T13105. This adds a (very basic) "Hexdump" engine (mostly just to have a second option to switch to) and a selector for choosing view modes.

Test Plan: Viewed some files, switched between audio/video/image/hexdump.

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19238