Added 'target' attribute to the allowed attributes array for link block

[facundocapua]

Added the 'target' property to the $allowedAttributes array, so it can be changed from the layout, like the following example:

 <block class="Magento\Framework\View\Element\Html\Link" name="a-blank-page-link">
    <arguments>
        <argument name="path" xsi:type="string">a-blank-page.html</argument>
        <argument name="target" xsi:type="string">_blank</argument>
        <argument name="label" xsi:type="string" translate="true">A blank page</argument>
    </arguments>
</block>

[magento-cicd2]

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Please see the CLA agreement in the Pull Request comments below.

[KrystynaKabannyk]

Hello @fcapua-summa, can you please update the code and cover it by Travis tests?

[facundocapua]

It seems that they are still failing, should I try restarting it again?

[vkorotun]

Internal ticket: MAGETWO-58372