Add pelican-page-hierarchy plugin

.gitmodules

@@ -34,3 +34,6 @@
 [submodule "pelican-page-order"]
 	path = pelican-page-order
 	url = git://github.com/akhayyat/pelican-page-order
+[submodule "pelican-page-hierarchy"]
+	path = pelican-page-hierarchy
+	url = git://github.com/akhayyat/pelican-page-hierarchy

pelican-page-hierarchy

@@ -0,0 +1 @@
+Subproject commit f11b533a50aa58800d1eade4aa6d3dfa17fd0f5c