Position parameter for era terms

[bwiernik]

The English "AD" term should generally come before the year, not after, though styles vary on this. In other languages, era terms may come before or after the year.

In CSL, historical era terms are placed after the year (e.g., 221 BC), whereas in Chinese convention they are placed before the year (e.g., 公元前 221 年). Therefore, these terms have been reverted to Latin forms, which are also acceptable in Chinese.

citation-style-language/locales#414

Should add support for a 'position' argument or similar to era terms to control where they are placed relative to the year.