Add custom syntax for tagging spoilers

Currently the only two ways to spoiler text in your message is either:
* Using the /spoiler command
* Manually typing the data-mx-spoiler span HTML blocks

Neither one is discoverable, or friendly to users really. Instead, we
should extend our existing Markdown-based formatting syntax with one
that can handle spoiler tags. I chose the || syntax to match Discord,
since Element doesn't seem to adopt one.

Unfortunately, CMark does not support custom extensions (see
https://github.com/commonmark/cmark/pull/123) so we have to implement
our own parsing function. New tests are also added for this too.

src/libneochat/texthandler.h

@@ -140,6 +140,7 @@ private:
     QString escapeHtml(QString stringIn);
     QString unescapeHtml(QString stringIn);
     QString linkifyUrls(QString stringIn);
+    QString customMarkdownToHtml(const QString &stringIn);
 
     QString editString() const;
     QString emoteString(const NeoChatRoom *room = nullptr, const Quotient::RoomEvent *event = nullptr) const;