<macro name="substitute-title"> for unsigned articles

Added <macro name="substitute-title"> to cope with unsigned articles and reviews, where the name of the newspaper or periodical (=container-title) needs to stand in place of the author (see CMoS 14.207 and 14.217). See discussion at https://forums.zotero.org/discussion/37961/choose-element-inside-substitute/
Umz, what @aurimas says is correct; I should be more careful.

A cs:substitute child forecloses execution of its subsequent siblings when any rendering node that it contains, or that are contained in any of its descendants, produces rendered output. So if container-title does not render anything, and the title macro follows it as the next child of cs:substitute, the title macro should execute.