Allow control of what virtual directories support Atom page feeds

A full discussion of this new feature is beyond the margins of this commit
message. See the discussion in
test/pages/dwiki/NewFeatures/AtomFeedsAndVirtualDirs.

Changed and added:
- atomgen.py: direct support for this
- dwconfig.py: configuration file support necessary for this
- test/pages/dwiki/ConfigurationFile: document new config directives
- test/pages/dwiki/NewFeatures/AtomFeedsAndVirtualDirs: full discussion