add support for links to the id of an element nested inside slide

[t-fritsch]

Following the issue #3231, this PR adds the possibility to link to a slide by the id of an element inside it (as stated in the issue, this is especialy useful when the markup is generated by an external tool).

Usage example :

<section>
    <h2>Slide 1</h2>
    <a href="#section-id">#section-id</a> (already working)<br/>
    <a href="#nested-id">#nested-id</a> (added by this PR)<br/>
</section>
<section id="section-id">
    <h2>Slide 2</h2>
    <div>
        <div id="nested-id">some div inside the slide</div>
    <div>
</section>

more complete example (with vertical slides)

<section>
	<a href="#section-id">#section-id</a> <br/>
	<a href="#nested-id">#nested-id</a> <br/>
	<a href="#vertical-section-id">#vertical-section-id</a> <br/>
	<a href="#vertical-nested-id-1">#vertical-nested-id-1</a> <br/>
	<a href="#vertical-section-id-2">#vertical-section-id-2</a> <br/>
	<a href="#vertical-nested-id-2">#vertical-nested-id-2</a> <br/>
</section>
<section id="section-id">
	<h2>section id</h2>
	<div id="nested-id"></div>
</section>
<section>
	<section id="vertical-section-id">
		<h2>vertical section id</h2>
		<ul>
			<li id="vertical-nested-id-1">
				vertical nested id 1
			</li>
			<li>
				<section>
					<li id="vertical-nested-id-2">
						vertical nested id 2
					</li>
				</section>
			</li>
		</ul>
	</section>
	<section id="vertical-section-id-2">
		<h2>vertical section id 2</h2>
	</section>
</section>

[hakimel]

Being able to link to any element by ID is a great addition. Thanks for adding that!