Add chicago page range variants (#380)

Based on discussion re: citation-style-language/documentation#115, add 
chicago-15 and chicago-16 page range formats.

Chicago changed its page range collapse rules in 16th edition. CSL 
currently follows 15th edition rules. This PR adds specific formats for 
the two versions. For 1.0.2, chicago will keep its current meaning and 
be an alias for chicago-15. In 1.1 and later, it will change to be an 
alias for chicago-16.

Author: bwiernik

.github/workflows/test.yaml

@@ -34,7 +34,7 @@ jobs:
     - name: Checkout Repository
       uses: actions/checkout@v2
     - name: Install dependencies
-      run: sudo apt-get install --yes trang
+      run: sudo apt-get update && sudo apt-get install --yes trang
     - name: Check schema validity
       run: trang schemas/styles/csl.rnc csl.rng && trang csl.rng csl.rnc
     - name: Check schema formatting

schemas/styles/csl.rnc

@@ -765,6 +765,8 @@ div {
       | "minimal-two"
       | "minimal-oup"
       | "chicago"
+      | "chicago-15"
+      | "chicago-16"
       | "mhra"
     }?
   citation.cite-group-delimiter =