Feature.editable

Add '.editable' support package:

- Declares an adapter interface, 'IEditable', for mapping content to / from a file-like representation.

- Provides an implementation for stock 'File' objects.

- Adds a configuration directive for registering new adapters.

- Declares another adapter interface 'ISource'.

- Applications can define an 'ISource' adapter, allowing the object to be edited as plain text / RST, or else as HTML.

- Provide a 'register_source_adapter' config directive for this adapter.  The directive *also* registers an 'IEditable' adapter for the context, using an implementation which depends on the semantics of 'ISource'.