Mustache.php v2.12.0

• Prevent redundant Autoloader registration (Thanks @hcpss-banderson!)
• Add a ProductionFilesystemLoader, which doesn't read template file contents before every render.
• Improve test coverage.
• Fix a bug when rendering the same block names multiple times in one template.
• Add a delimiters option for overriding default delimiters at the engine level.
• Add validation to prevent empty template_class_prefix config.