Generalized support for handling content types in spheres

[cdata]

Currently, spheres mainly consist of a single content type: text/subtext. However, in the fullness of time we want Noosphere to be an appealing storage and exchange layer for arbitrary content. This includes rich content types like images, videos, 3D models etc., as well as other text-based content types or really anything.

In pursuit of this goal, we may leverage a variety of techniques. One that seems particularly appealing is transformation of data as it is being read via something like autocodecs. There are probably other interesting paths to consider as well.