Implemented Sieve extracttext extension.

Makefile.am

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

src/lib-sieve/plugins/mime/cmd-extracttext.c

src/lib-sieve/plugins/mime/ext-extracttext.c

src/lib-sieve/plugins/mime/ext-mime-common.h

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

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

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

src/lib-sieve/sieve-extensions.c

tests/extensions/mime/errors.svtest

tests/extensions/mime/errors/extracttext-nofep.sieve

tests/extensions/mime/errors/extracttext-novar.sieve

tests/extensions/mime/errors/extracttext.sieve

tests/extensions/mime/extracttext.svtest