Gruntfile: skip steps when $TWBS_DO_*=0

Gruntfile.js

@@ -404,6 +404,9 @@ module.exports = function (grunt) {
   var runSubset = function (subset) {
     return !process.env.TWBS_TEST || process.env.TWBS_TEST === subset;
   };
+  var isUndefOrNonZero = function (val) {
+    return val === undefined || val !== '0';
+  };
 
   // Test task.
   var testSubtasks = [];
@@ -412,13 +415,17 @@ module.exports = function (grunt) {
     testSubtasks = testSubtasks.concat(['dist-css', 'csslint', 'jshint', 'jscs', 'qunit', 'build-customizer-html']);
   }
   // Skip HTML validation if running a different subset of the test suite
-  if (runSubset('validate-html')) {
+  if (runSubset('validate-html') &&
+      // Skip HTML5 validator on Travis when [skip validator] is in the commit message
+      isUndefOrNonZero(process.env.TWBS_DO_VALIDATOR)) {
     testSubtasks.push('validate-html');
   }
   // Only run Sauce Labs tests if there's a Sauce access key
   if (typeof process.env.SAUCE_ACCESS_KEY !== 'undefined' &&
       // Skip Sauce if running a different subset of the test suite
-      runSubset('sauce-js-unit')) {
+      runSubset('sauce-js-unit') &&
+      // Skip Sauce on Travis when [skip sauce] is in the commit message
+      isUndefOrNonZero(process.env.TWBS_DO_SAUCE)) {
     testSubtasks.push('connect');
     testSubtasks.push('saucelabs-qunit');
   }