Variables: added dumptime context.

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

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-dump.c

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

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

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

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