Wiki-style annotations / collaborative authorship

[JakeHartnell]

Moving this over here from https://github.com/hypothesis/h/issues/300

@dwhly

Based on our discussions w/ Michael Shavlovsky and Luca de Alfaro, and our sense that the "RapGenius" model of annotation probably has a role to play.

This google doc discusses this towards the end... https://docs.google.com/a/hypothes.is/document/d/1EK785Rjdrw-so2kPsuh1T38xUVkbNzFbb3NLKBfjge0/edit#

[dwhly]

This is also closely related to #84.