Variables extension: added basic namespace support.

TODO

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

src/lib-sieve/plugins/variables/ext-variables-arguments.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-modifiers.h

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

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

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

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

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

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

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

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