(hack-dir-local-get-variables-functions): New hook

(hack-dir-local-get-variables-functions): New hook

Make it possible to provide more dir-local variables, such as
done by the Editorconfig package.

* lisp/files.el (hack-dir-local--get-variables): Make arg optional.
(hack-dir-local-get-variables-functions): New hook.
(hack-dir-local-variables): Run it instead of calling
`hack-dir-local--get-variables`.

* doc/lispref/variables.texi (Directory Local Variables):
Document the new hook.