Do not escape some allowed tags in titles

[glorieux-f]

In CSL spec, some tags are allowed in complex title.
https://citeproc-js.readthedocs.io/en/latest/csl-json/markup.html#html-like-formatting-tags

Consider this forged example

Read <i><i>Laissez-Faire</i> Banking</i> in the <span style=\"font-variant:small-caps;\">xxi</span><sup>st</sup>

It is allowed and should be correctly handled by a CSL processor. Zotero do that in javascript:
https://github.com/zotero/zotero/blob/408f1274f4d98b72204393dd1392d71d6e7d507e/chrome/content/zotero/xpcom/utilities_internal.js#L2448-L2565

Citeproc-php has for now adopted a cautious approach, escape all HTML entities. Nothing is lost, but HTML display could be frustrating. Fortunately, the code is well architected, there is only one line to improve.

citeproc-php/src/Rendering/Text.php

Lines 237 to 242 in 56abb9c

	return $this->applyTextCase(
	StringHelper::clearApostrophes(
	htmlspecialchars($value, ENT_HTML5)
	),
	$lang
	);

By the way, why call StringHelper::clearApostrophes() here ? It is also done at a Layout level.
https://github.com/seboettg/citeproc-php/blob/develop/src/Rendering/Layout.php
What is the technical constraint to modify the user input ?

User could be happy to choose between more than one kind of text output. Because of historic reason, htmlspecialchars($value, ENT_HTML5) should be the default, but overriding with a custom function should be possible.

2 commits on src/Rendering/Text.php will follow for this issue.

[glorieux-f]

With this commit, customizing text output is possible with a Callable.

   $additionalMarkup = [
     "text" => 'Seboettg\CiteProc\Rendering\Text::renderTextRich',
   ]