Added variable operand to the variables extension.

src/lib-sieve/plugins/variables/cmd-set.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-actions.c

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

src/lib-sieve/sieve-code.c

src/lib-sieve/sieve-code.h

src/lib-sieve/sieve-comparators.c

src/lib-sieve/sieve-match-types.c