Quick solution for ionic build optimized

Ionic production build

Step 1: Add some npm packages to your devDependencies section in package.json:

"gulp-usemin": "^0.3.8",
"gulp-util": "^2.2.14",
"run-sequence": "^1.0.2",
"shelljs": "^0.3.0"

Step 2: Import them in gulpfile.js:

var sh = require('shelljs');
var usemin = require('gulp-usemin');
var runSequence = require('run-sequence');
var fs = require('fs');
var karma = require('karma').server;

Step 3: Add some paths

var paths = {
  sass: ['./scss/**/*.scss'],
  tempWww: './www-tmp',
  filesToCopy: [
    'www/index.html',
    'www/img/**/*',
    'www/locales/**/*',
    'www/templates/**/*',
    'www/lib/ionic/fonts/ionicons.woff',
    'www/css/*'
  ]
};

Step 4: Add these new tasks to gulpfile.js:

gulp.task('build-ionic', function(done) {
  if (!fs.existsSync(paths.tempWww)) {
    fs.mkdirSync(paths.tempWww);
  }
  runSequence('sass', 'usemin', 'pack', 'stash-www', 'call-ionic', 'unstash-www');
});

gulp.task('call-ionic', function() {
  sh.exec('ionic build');
});

gulp.task('stash-www', function() {
  if (!fs.existsSync('./www.bak')) {
    fs.renameSync('./www', './www.bak');
    fs.renameSync(paths.tempWww, './www');
  }
});

gulp.task('unstash-www', function() {
  if (fs.existsSync('./www.bak')) {
    deleteFolderRecursive('www');
    fs.renameSync('./www.bak', './www');
  }
});

gulp.task('pack', function(done) {
  if (!fs.existsSync(paths.tempWww)) {
    fs.mkdirSync(paths.tempWww);
  }
  
  gulp.src(paths.filesToCopy, { base: 'www/' })
    .pipe(gulp.dest(paths.tempWww))
    .on('end', done);
});

gulp.task('usemin', function() {
  gulp.src('./www/index.html')
    .pipe(usemin({
      // you could add some other tasks here, like uglify, I was in a hurry and didn't really need it
    }))
    .pipe(gulp.dest(paths.tempWww));
});

/** Credit http://stackoverflow.com/a/12761924/2768444 */
function deleteFolderRecursive(path) {
    var files = [];
    if( fs.existsSync(path) ) {
        files = fs.readdirSync(path);
        files.forEach(function(file,index){
            var curPath = path + "/" + file;
            if(fs.lstatSync(curPath).isDirectory()) { // recurse
                deleteFolderRecursive(curPath);
            } else { // delete file
                fs.unlinkSync(curPath);
            }
        });
        fs.rmdirSync(path);
    }
}

Some notes:
- I had to use runSequence because otherwise usemin would end after sass and the script tags would not be replaced, it's a quick and dirty solution
- I don't know of a way to tell cordova to use other dir than www for source,
  so I moved the source dir out of the way and replaced it with the temporary dir
  IMPORTANT: make sure you have your project in a git repo and you have commited your work, in case something wrong happens 
              and your source www dir will get removed

Step 5: Add the the usemin tags in your index.html (this is how I did it, you can group your sources any way you see fit)

<!-- build:js js/lib.js -->
<script src="lib/ionic/js/ionic.bundle.js"></script>
<!-- other libs you might use -->
<!-- endbuild -->

<!-- build:js js/scripts.js -->
<!-- your scripts in www/js -->
<!-- endbuild -->

Step 6: Call your new task: gulp ionic-build