Update to reflect the current status of the configuration system.

Update to reflect the current status of the configuration system.
Extensions must still be configured manually and there is currently one
set of extension key bindings for all platforms.

Bring NEWS.txt up to date.

Update CREDITS.txt and idlever.py for release.

M NEWS.txt
M config-extensions.def
M extend.txt
M help.txt
M idlever.py