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/parse5/lib/utils/merge_options.js

@@ -0,0 +1,13 @@
+'use strict';
+
+module.exports = function mergeOptions(defaults, options) {
+    options = options || Object.create(null);
+
+    return [defaults, options].reduce(function (merged, optObj) {
+        Object.keys(optObj).forEach(function (key) {
+            merged[key] = optObj[key];
+        });
+
+        return merged;
+    }, Object.create(null));
+};

node_modules/parse5/lib/utils/mixin.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var Mixin = module.exports = function (host) {
+    var originalMethods = {},
+        overriddenMethods = this._getOverriddenMethods(this, originalMethods);
+
+    Object.keys(overriddenMethods).forEach(function (key) {
+        if (typeof overriddenMethods[key] === 'function') {
+            originalMethods[key] = host[key];
+            host[key] = overriddenMethods[key];
+        }
+    });
+};
+
+Mixin.prototype._getOverriddenMethods = function () {
+    throw new Error('Not implemented');
+};
+