task alias convenience method

It is very common to declare task aliases such as `default` and maybe for each different workflow you have.

Before:

```js
gulp.task('default', function () {
  gulp.run('jshint', 'build');
});
gulp.task('build', function () {
  gulp.run('js', 'css', 'copy');
});
```

After:

```js
gulp.alias('default', 'jshint build');
gulp.alias('build', 'js css copy');
```

This is a feature which was always missing in Grunt and which would be nice to add in Gulp. It doesn't add any complexity. I guess it's a nice to have.

Thoughts?