util: Introduce isFileValid() and itTimeValid() convenient utilit…

…y functions

The are used in `bem-www` `.bem/make.js`, for example

lib/util.js

@@ -263,6 +263,42 @@ exports.findLevel = function(path, startPath) {
     return exports.findLevel(PATH.dirname(path), startPath);
 };
 
+/**
+ * Checks if `target` file is valid comparing it's mtime
+ * with all then `depends` files.
+ *
+ * @param {String} target  Path to file to check validity status.
+ * @param {String[]|String} depends  Target file dependencies to compare mtime to.
+ * @return {Boolean}
+ */
+exports.isFileValid = function(target, depends) {
+
+    Array.isArray(depends) || (depends = [depends]);
+    return Q.all([
+            QFS.lastModified(target)
+                .fail(function() {
+                    return -1;
+                }),
+            Q.all(depends.map(QFS.lastModified.bind(QFS)))
+        ])
+        .spread(exports.isTimeValid);
+
+};
+
+/**
+ * Checks if `target` if the largest number among `depends`.
+ *
+ * Useful to compare timestamps.
+ *
+ * @param {Number} target
+ * @param {Number[]|Number} depends
+ * @return {Boolean}
+ */
+exports.isTimeValid = function(target, depends) {
+    Array.isArray(depends) || (depends = [depends]);
+    return target >= Math.max.apply(Math, depends);
+};
+
 /**
  * Filter out non-existent paths.
  *