feat: Add a default filter

Sources/Filters.swift

@@ -31,3 +31,7 @@ func lowercase(_ value: Any?) -> Any? {
 
   return value
 }
+
+func defaultFilter(value: Any?, arguments: [Any?]) -> Any? {
+  return value ?? arguments.first as Any
+}

Sources/Namespace.swift

@@ -45,6 +45,7 @@ public class Namespace {
   }
 
   fileprivate func registerDefaultFilters() {
+    registerFilter("default", filter: defaultFilter)
     registerFilter("capitalize", filter: capitalise)
     registerFilter("uppercase", filter: uppercase)
     registerFilter("lowercase", filter: lowercase)