Added basic extension support to generator and interpreter.

.hgignore

src/lib-sieve/Makefile.am

src/lib-sieve/cmd-require.c

src/lib-sieve/ext-envelope.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/scripts/tests/reject.sieve

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-binary.c

src/lib-sieve/sieve-binary.h

src/lib-sieve/sieve-code.h

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/lib-sieve/sieve-generator.c

src/lib-sieve/sieve-generator.h

src/lib-sieve/sieve-interpreter.c

src/lib-sieve/sieve-interpreter.h

src/lib-sieve/sieve-validator.c

src/lib-sieve/sieve-validator.h

src/lib-sieve/sievec.c