title: Specifying unsigned attribute when using the Schema Builder Trait in MIgrations?
body: <p>I have to specify a column as unsigned in yii2 migrations .<br>
  Example migration code from manual</p>
  
  <pre><code>public function up()
  {
      $this-&gt;createTable('news', [
          'id' =&gt; $this-&gt;primaryKey(),
          'title' =&gt; $this-&gt;string()-&gt;notNull()
      ]);
  }
  </code></pre>
  
  <p>From the research I have done there doesn't seem to be a method to add the unsigned capability in schema builder trait. </p>
  
  <p>But is there some other way I can add unsigned attribute to the column while still making use of the schemaBuilderTrait style methods ?</p>
  
  <p>For instance the <code>$this-&gt;string()</code> above returns an instance of <code>yii\db\ColumnSchemaBuilder</code>, but that doesn't even have a property to set unsigned/signed.. </p>