mkdocs: include additional hljs languages.

We upgraded mkdocs earlier to support some useful features (like the
mkdocs-exclude plugin), but that one changes the highlight.js
configuration to include fewer languages by default.  Specify the
additional ones we want to include.

Reported-by: @DRMacIver on twitter

mkdocs.yml

@@ -1,5 +1,11 @@
 site_name: "redo: a recursive build system"
-theme: readthedocs
+theme:
+  name: readthedocs
+  hljs_languages:
+    - python
+    - r
+    - shell
+    - tex
 docs_dir: docs
 site_dir: docs.out
 strict: true