Further developed the variables extension.

src/lib-sieve/plugins/variables/Makefile.am

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/plugins/variables/tst-string.c

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