Add support for adapters

[timlehr]

Currently Raven doesn't support adapters for AAF, XML, etc. Having support for non-OTIO read / write operations would be really great. Here are some ideas on how to implement it:

• subprocess pipe to otioconvert
• add an embedded Python install

Constraint: We want to ensure this tool stays light weight, and works in the browser.

[jminor]

Capturing some offline discussion: we would prefer to use a subprocess pipe to otioconvert instead of adding a Python dependency to Raven. A big part of Raven's appeal and success so far is the ease of building it, which an added Python dependency would complicate. If however, Raven could find (or be configured with) a path to a working otioconvert then it could run that utility to convert between OTIO and other formats.