<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/
I know that @fbennett specifically mentioned this on the Zotero forums, but I don't understand why this is necessary (not to say that it's not). If container-title is empty, then title would just be the next fallback in the substitute anyway, so I think that what you proposed originally would have worked the same way. I.e.
```
<macro name="substitute-title">
  <choose>
    <if type="article-journal article-magazine article-newspaper" match="any">
      <text macro="container-title"/>
    </if>
  </choose>
</macro>
...
<substitute>
  <names variable="editor"/>
  <names variable="translator"/>
  <text macro="substitute-title"/>
  <text macro="title"/>
</substitute>
```
(though maybe better name for that macro is appropriate)