feat(grunt): add autoprefix with postcss

closes #4250

tasks/default_task.js

@@ -2,7 +2,16 @@
 module.exports = function(grunt) {
   'use strict';
 
-  grunt.registerTask('css', ['sass', 'concat:cssDark', 'concat:cssLight', 'styleguide', 'sasslint']);
+  grunt.registerTask('css', [
+    'sass',
+    'concat:cssDark',
+    'concat:cssLight',
+    'styleguide',
+    'sasslint',
+    'postcss'
+    ]
+  );
+
   grunt.registerTask('default', [
     'jscs',
     'jshint',

tasks/options/postcss.js

@@ -0,0 +1,21 @@
+module.exports = function(config) {
+
+  return {
+    options: {
+      map: true, // inline sourcemaps
+
+      // or
+      map: {
+        inline: false, // save all sourcemaps as separate files...
+        annotation: 'dist/css/maps/' // ...to the specified directory
+      },
+
+      processors: [
+        require('autoprefixer')({browsers: 'last 2 versions'}), // add vendor prefixes
+      ]
+    },
+    dist: {
+      src: '<%= genDir %>/css/*.css'
+    }
+  }
+};