add beginnings of a common place for all standard extensions

src/clj_pebble/standard_extensions.clj

@@ -0,0 +1,13 @@
+(ns clj-pebble.standard-extensions)
+
+(def extensions
+  {:functions
+    {"concat"
+      (fn [& values]
+        (apply str values))}
+
+   :filters
+    {}
+
+   :tests
+    {}})