Added $url property to Button. Its allow to render <a href..> as buttons...

.... It is possible already but is not too usable in some cases. For example, if we need to render a buttongroup with href buttons and iconified labels(encodeLabels = false), we write something like this:

~~~
                echo ButtonGroup::widget([
                    'buttons' => [
                        Button::widget([
                            'tagName' => 'a',
                            'encodeLabel' => false,
                            'label' => Icon::show('icon') . Yii::t('app', 'Index'),
                            'options' => [
                                'href' => Url::toRoute(['controller/index']),
                            ],
                        ]),
                        Button::widget([
                            'tagName' => 'a',
                            'encodeLabel' => false,
                            'label' => Icon::show('plus'),
                            'options' => [
                                'href' => Url::toRoute(['controller/create']),
                            ],
                        ]),
                    ]
                ]);
~~~

Be noted, we cannot use button's configs since $tagName(needed to render 'a' tag) is not used in ButtonGroup::renderButtons... so we need to define 'encodeLabel' for each button.

With this patch we can write something like this:

~~~
                echo ButtonGroup::widget([
                    'encodeLabels' => false,
                    'buttons' => [
                        [
                            'label' => Icon::show('icon') . Yii::t('app', 'Index'),
                            'url' => ['controller/index'],
                        ],
                        [
                            'label' => Icon::show('plus'),
                            'url' => ['controller/create'],
                        ],
                    ]
                ]);
~~~