implement option to use relative markdown links

[secure-77]

classic markdown links are all currently handled as external links, so internal / relative links are not working.
We can fix this by converting all markdown links (which not contain an :) to obsidian format before rendering

in the content.php before Parsedown is called:

    // replace internal markdown links to obsidian
	$replaces = '[[\\2|\\1]]';
	$pattern = array('/\[(.*?)\]\((?![^)]*?:)([^)]+)\)/');
	$content = preg_replace($pattern, $replaces, $content);