oo3 extension used for different file formats

[sophos at mailcan.com]

Version 3 is using the same file name extension, .oo3, for documents 
stored as pure xml, zip compressed xml, and for the directory that 
wraps documents with attachments.  While I like having all of these 
options, it makes taking advantage of the benefits of xml more 
difficult.  For other applications to handle the .oo3 files, they need 
to know more than just the extension to parse the xml, they must have 
additional OmniOutliner specific implementation knowledge to 
discriminate between the different document formats.

Now that OOv3 is released, it is probably too late to change, but it 
would be nice to have the option to use different file extensions for 
the different file formats.  Perhaps something like,

.oo3  for the current behavior
.o3x  for xml text
.o3z  for zip compressed
.o3a  for documents with attachments