Make nested list compliant with w3c standards

[duylecampos]

The getListMarkup doesn't convert the draftjs block to a w3c compliant nested list.

Transformation result

<ul>
    <li>1</li>
    <li>2</li>
    <ul>
        <li>2-1</li>
        <li>2-2</li>
    </ul>
    <li>3</li>
</ul>

Expected W3C compliant

<ul>
    <li>1</li>
    <li>2
        <ul>
            <li>2-1</li>
            <li>2-2</li>
        </ul>
    </li>
    <li>3</li>
</ul>

A validation can be done on https://validator.w3.org/nu/#textarea
Here the result using the library output: