Re-add Replacer plugin as submodule. Refs #444.

.gitmodules

@@ -70,3 +70,6 @@
 [submodule "pelican-open_graph"]
 	path = pelican-open_graph
 	url = https://github.com/whiskyechobravo/pelican-open_graph.git
+[submodule "replacer"]
+	path = replacer
+	url = https://github.com/narusemotoki/replacer

replacer

@@ -0,0 +1 @@
+Subproject commit f123acbd700ea8f2c40552d0746f8459ef006b25