Introduce variable filters

Stencil/Filters.swift

@@ -0,0 +1,33 @@
+func toString(value: Any?) -> String? {
+  if let value = value as? String {
+    return value
+  } else if let value = value as? CustomStringConvertible {
+    return value.description
+  }
+
+  return nil
+}
+
+func capitalise(value: Any?) -> Any? {
+  if let value = toString(value) {
+    return value.capitalizedString
+  }
+
+  return value
+}
+
+func uppercase(value: Any?) -> Any? {
+  if let value = toString(value) {
+    return value.uppercaseString
+  }
+
+  return value
+}
+
+func lowercase(value: Any?) -> Any? {
+  if let value = toString(value) {
+    return value.lowercaseString
+  }
+
+  return value
+}