/// <summary>
/// Includes a custom extension in the markdown processing given by a class implementing
/// the IMarkdownExtension interface.
/// </summary>
/// <typeparam name="TExtension">The type of the extension to use.</typeparam>
public Markdown UseExtension <TExtension>()
where TExtension : class, IMarkdownExtension, new()
{
_extensions.AddIfNotAlready <TExtension>();
return(this);
}