Missing parts in link_attributes extension support

[Omikhleia]

For recollection and possible future improvement, the link_attributes extension support added in #36 is partial: it is supported on direct images only... but not on links and indirect images.

For parity with Pandoc, these should be supported:

On [link](#target){ #source .class key=value }

On [indirect link][reflink]

[reflink]: #target { #source .class key=value }

On ![indirect image][imgref]

[imgref]: image.png "opt title" {#id .class key=value}

Generating, e.g. in HTML, something such as:

<p>On <a href="#target" id="source" class="class" data-key="value">link</a></p>
<p>On <a href="#target" id="source" class="class" data-key="value">indirect link</a></p>
<p>On <img src="image.png" title="opt title" id="id" class="class" data-key="value" alt="indirect image" /></p>

[Witiko]

Furthermore, we should also support attributes on autolinks:

<mailto:some@one.name>{.family}

In HTML, this should produce the following output:

<p><a href="mailto:some@one.name" class="family">mailto:some@one.name</a></p>