Add henceforth term (#296)

Used for styles that switch to an abbreviated `citation-label` format for subsequent citations.

Long and short forms might be localized as "henceforth cited as" and "henceforth" respectively.

Closes citation-style-language/csl-evolution#37

Author: bwiernik

schemas/styles/csl-terms.rnc

@@ -34,6 +34,7 @@ div {
     | "film"
     | "forthcoming"
     | "from"
+    | "henceforth"
     | "ibid"
     | "in"
     | "in press"