Made basic execution implementation of the envelope extension.

sieve/tests/envelope.sieve

src/lib-sieve/ext-envelope.c

src/lib-sieve/sieve-address-parts.c

src/lib-sieve/sieve-address-parts.h

src/lib-sieve/sieve.h

src/lib-sieve/tst-address.c

src/sieve-bin/sieve_test.c