jinja2.ext.Extension

Extensions can be used to add extra functionality to the Jinja template system at the parser level. Custom extensions are bound to an environment but may not store environment specific data on `self`. The reason for this is that an extension can be bound to another environment (for overlays) by creating a copy and reassigning the `environment` attribute. As extensions are created by the environment they cannot accept any arguments for configuration. One may want to work around that by using a factory function, but that is not possible as extensions are identified by their import name. The correct way to configure the extension is storing the configuration values on the environment. Because this way the environment ends up acting as central configuration storage the attributes may clash which is why extensions have to ensure that the names they choose for configuration are not too generic. ``prefix`` for example is a terrible name, ``fragment_cache_prefix`` on the other hand is a good name as includes the name of the extension (fragment cache).