Major rework of envelope address handling: a normalized version of the envelope addresses is maintained in the message context and message context is now also available during action execution.

Added compilation support for ereject extension. It is unfinished right now and performs exactly the same action as reject.

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.

Improved error message for unknown Sieve extension to account for core commands included as an extension.