Store readonly/nullable in attribute idl markup, and insert this information in <span attribute-info> spans.

The attribute currentTime of AnimationTimeline:
<code>
interface AnimationTimeline {
    readonly attribute double? currentTime;
}
</code>

was marked up as:
<code>
&lt;a title="currentTime" data-idl-for=" data-idl-type="attribute"&gt;currentTime&lt;/a&gt;
</code>

I've added data-type and an optional data-readonly:
<code>
&lt;a title="currentTime" data-idl-for=" data-idl-type="attribute" data-readonly data-type="double?" &gt;currentTime&lt;/a&gt;
</code>

I've also added processing of
<code>
&lt;span attribute-info for=currentTime&gt;&lt;/span&gt;
</code>

To produce
<code>
&lt;span data-attribute-info for="currentTime"&gt; of type &lt;span data-type&gt;double&lt;/span&gt;, readonly, nullable&lt;/span&gt;
</code>