Note: No backwards-compatability layer is included for manual inclusion
of filter.php as this is not a public API. Inclusion should have always
been through the filter manager, and the filter's own unit tests.
If enabled, the filter will convert text wrapped by language-* style class
into a well-styled block of code.
The filter uses PrismJS, and it also used by the TinyMCE editor to
highlight the code sample plugin.