Add m.censor, adjust m.route.Link to use it (#2538)

Also, restructure a few things for better code reuse.

util/assign.js

@@ -0,0 +1,10 @@
+// This exists so I'm only saving it once.
+"use strict"
+
+var hasOwn = require("./hasOwn")
+
+module.exports = Object.assign || function(target, source) {
+	for (var key in source) {
+		if (hasOwn.call(source, key)) target[key] = source[key]
+	}
+}