Add support for authoring custom rules.

lib/shared.js

@@ -46,6 +46,16 @@ module.exports.clone = function clone(obj) {
   return assign({}, obj);
 };
 
+// Returns true iff the input is a string
+module.exports.isString = function isString(obj) {
+  return typeof obj === "string";
+};
+
+// Returns true iff the input string is empty
+module.exports.isEmptyString = function isEmptyString(str) {
+  return str.length === 0;
+};
+
 // Replaces the text of all properly-formatted HTML comments with whitespace
 // This preserves the line/column information for the rest of the document
 // Trailing whitespace is avoided with a '\' character in the last column