enable linked years for citations and create a section heading for references

aa/demo-structured_abstract.md

@@ -1,5 +1,6 @@
 ---
 preambleinput: ["macros.tex"]
+link-citations: true
 texpackages:
 - name: xspace
 #  opts:
@@ -40,3 +41,5 @@ G.C.P. acknowledges support from the University of Florida.
 This work makes us of the `pandoc` universal document convertor^[<https://pandoc.org/>].
 
 {{end_acknowledgments}}
+
+# References

aa/demo-traditional_abstract.md

@@ -1,5 +1,6 @@
 ---
 preambleinput: ["macros.tex"]
+link-citations: true
 texpackages:
 - name: xspace
 #  opts:
@@ -35,3 +36,5 @@ G.C.P. acknowledges support from the University of Florida.
 This work makes us of the `pandoc` universal document convertor^[<https://pandoc.org/>].
 
 {{end_acknowledgments}}
+
+# References

aastex62/demo.md

@@ -1,6 +1,7 @@
 ---
 aastexopts: [singlecolumn]
 preambleinput: ["macros.tex"]
+link-citations: true
 texpackages:
 - name: xspace
 #  opts:
@@ -313,3 +314,5 @@ Comments, bug reports, and enhancements are welcome.
 {{acknowledgments}}
 
 I thank Kelly Hondula for comments on this manuscript and acknowledge support from the University of Florida.
+
+# References

mnras/demo.md

@@ -1,5 +1,6 @@
 ---
 preambleinput: ["macros.tex"]
+link-citations: true
 texpackages:
 - name: xspace
 #  opts:
@@ -34,3 +35,5 @@ See the aastex demonstration^[<https://github.com/privong/papers-in-markdown/tre
 
 G.C.P. acknowledges support from the University of Florida.
 This work makes us of the `pandoc` universal document convertor^[<https://pandoc.org/>].
+
+# References