Setting up a new highlighter object is slightly expensive since it
reads and parses the langfile each time. So cache them.
This also speeds up ext2langfile by avoiding it needing to check for the
existence of a language file in some cases.
format: Provide a htmlizefallback hook that other plugins can use to
handle formats that are not suitable for general-purpose htmlize hooks.
highlight: Use the hook to allow formatting of any language/extension,
without it needing to be enabled for standalone source files.
highlight: If the highlight perl binding is not available, fallback
safely to a passthrough mode.
Joey Hess