Added support for per-script extension intialization.

src/lib-sieve/ext-encoded-character.c

src/lib-sieve/ext-envelope.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/body/ext-body.c

...gins/comparator-i-ascii-numeric/ext-cmp-i-ascii-numeric.c

src/lib-sieve/plugins/copy/ext-copy.c

src/lib-sieve/plugins/imapflags/ext-imapflags.c

src/lib-sieve/plugins/include/ext-include.c

src/lib-sieve/plugins/regex/ext-regex.c

src/lib-sieve/plugins/relational/ext-relational.c

src/lib-sieve/plugins/subaddress/ext-subaddress.c

src/lib-sieve/plugins/vacation/ext-vacation.c

src/lib-sieve/plugins/variables/ext-variables-arguments.c

src/lib-sieve/plugins/variables/ext-variables-common.c

src/lib-sieve/plugins/variables/ext-variables-common.h

src/lib-sieve/plugins/variables/ext-variables.c

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

src/lib-sieve/sieve-code-dumper.c

src/lib-sieve/sieve-comparators.c