Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/lodash/_cloneRegExp.js

@@ -0,0 +1,17 @@
+/** Used to match `RegExp` flags from their coerced string values. */
+var reFlags = /\w*$/;
+
+/**
+ * Creates a clone of `regexp`.
+ *
+ * @private
+ * @param {Object} regexp The regexp to clone.
+ * @returns {Object} Returns the cloned regexp.
+ */
+function cloneRegExp(regexp) {
+  var result = new regexp.constructor(regexp.source, reFlags.exec(regexp));
+  result.lastIndex = regexp.lastIndex;
+  return result;
+}
+
+module.exports = cloneRegExp;