Testsuite: added tests for envelope/address test behavior with non-existant subaddress :detail part.

Envelope: fixed bug in application of address parts; failure to obtain the part would cause inappropriate match success (bug reported by Ron Lee)

Made deprecated notify extension implementation compatible with cmusieve, except for the denotify command.

Fixed validator extension validation. It validated the first non-require command before validating the extensions, which produced useless error messages.

Enhanced extensions configuration, allowing to specify the enabled extensions relatively to the default.