add rel link attribute

[dpprdan]

This PR adds a rel attribute to links. For example, rel="me" is used by Mastodon for link verification.

I changed the Twitter links to Mastodon for illustration (it also shows how to add the Mastodon user name with @, cf. #57).

See it in action on my site (mastodon, source).

I noticed that there are two div blocks for the links in the pandoc templates. For example in the jolla template here:

postcards/inst/pandoc_templates/jolla.html

Lines 50 to 64 in 8f92e4d

	<div class="row justify-content-center align-items-center">
	<div class="col-8 text-center d-none d-sm-block">
	<ul class="list-inline">
	$for(links)$
	<li class="list-inline-item">
	<a href=$links.url$>
	<button type="button" class="btn btn-outline-dark mb-2">
	$links.label$
	</button>
	</a>
	</li>
	$endfor$
	</ul>
	</div>
	</div>

and here

postcards/inst/pandoc_templates/jolla.html

Lines 66 to 77 in 8f92e4d

	<div class="row justify-content-center align-items-center">
	<div class="col-8 m-1 text-center d-block d-sm-none">
	<div class="list-group">
	$for(links)$
	<a href=$links.url$>
	<button type="button" class="list-group-item list-group-item-action my-1 rounded">
	$links.label$
	</button>
	</a>
	$endfor$
	</div>
	</div>

Is it necessary to have both and if yes, what for?