Backwards compatibility is provided for vnd.dovecot.duplicate.
Still need to fix the constraint that it must not track duplicates from failed Sieve executions.

It is still a plugin, but no longer a separate package.

It is now possible to track duplicates based on arbitrary headers or even arbitrary string values using the new :header and :value arguments respectively.
The experation time can be configured using the new :seconds argument.
This change is backwards compatible as long as the name argument wasn't used. This is now a :handle <handle> argument.

Added support for the :optional tag of the include command.
Changed Sieve script API in the process; opening the script is now available as a separate step.

Also fixed a small mistake in the spefication for the vnd.dovecot.duplicate extension.

Incorporated sieve_duplicate plugin into main Pigeonhole tree as a normal extension (vnd.dovecot.duplicate).
Also restructured doc dir a bit in the process.

Variables: implemented limits on number of variables in a scope, the length of variable names, size of variable values and the number of accesible match values.