BibTeX writer fixes. Closes #7067.

+ Require citeproc 0.3.0.7, which correctly titlecases when titles
  contain non-ASCII characters.
+ Correctly handle 'pages' (= 'page' in CSL).
+ Correctly handle BibLaTeX 'langid' (= 'language' in CSL).
+ In BibTeX output, protect foreign titles since there's no language
  field.

1 files changed, 90 insertions, 0 deletions

diff --git a/test/command/7067.md b/test/command/7067.md
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+```
+% pandoc -t biblatex
+---
+references:
+- id: garaud
+  author:
+    - family: Garaud
+      given: Marcel
+  container-title: Bulletin de la Societé des antiquaires de l’Ouest
+  collection-title: 4
+  issued:
+    - year: 1967
+  language: fr-FR
+  page: 11-27
+  title: Recherches sur les défrichements dans la Gâtine poitevine aux XI^e^ et XII^e^ siècles
+  type: article-journal
+  volume: 9
+...
+^D
+@article{garaud,
+  author = {Garaud, Marcel},
+  title = {Recherches sur les défrichements dans la Gâtine poitevine aux
+XI\textsuperscript{e} et XII\textsuperscript{e} siècles},
+  journal = {Bulletin de la Societé des antiquaires de l’Ouest},
+  series = {4},
+  volume = {9},
+  pages = {11-27},
+  date = {1967},
+  langid = {fr-FR}
+}
+```
+
+```
+% pandoc -t bibtex
+---
+references:
+- id: garaud
+  author:
+    - family: Garaud
+      given: Marcel
+  container-title: Bulletin de la Société des antiquaires de l’Ouest
+  collection-title: 4
+  issued:
+    - year: 1967
+  language: fr-FR
+  page: 11-27
+  title: Recherches sur les défrichements dans la Gâtine poitevine aux XI^e^ et XII^e^ siècles
+  type: article-journal
+  volume: 9
+...
+^D
+@article{garaud,
+  author = {Garaud, Marcel},
+  title = {{Recherches sur les défrichements dans la Gâtine poitevine aux
+XI\textsuperscript{e} et XII\textsuperscript{e} siècles}},
+  journal = {Bulletin de la Société des antiquaires de l’Ouest},
+  series = {4},
+  volume = {9},
+  pages = {11-27},
+  year = {1967}
+}
+```
+
+This tests the titlecasing of a word with an accented second letter:
+```
+% pandoc -t bibtex
+---
+references:
+- id: garaud
+  author:
+    - family: Garaud
+      given: Marcel
+  container-title: English Journal
+  issued:
+    - year: 1967
+  language: en-US
+  title: Research on the défrichements in the Gâtine poitevine
+  type: article-journal
+  volume: 9
+...
+^D
+@article{garaud,
+  author = {Garaud, Marcel},
+  title = {Research on the Défrichements in the {Gâtine} Poitevine},
+  journal = {English Journal},
+  volume = {9},
+  year = {1967}
+}
+```
+