Implemented support for configuring the available extensions.

TODO

doc/man/sieve-test.1

doc/man/sievec.1

src/lib-sieve-tool/sieve-tool.c

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/lib-sieve/sieve.c

src/lib-sieve/sieve.h

src/plugins/lda-sieve/lda-sieve-plugin.c

src/sieve-tools/sieve-test.c

src/sieve-tools/sievec.c