Introduce dynamic filtering for repository folders

Allow people who want folder filtering to depend on the external
conditions or to make it dynamic for other reasons, to do what
they want.

New repository configuration knob 'dynamic_folderfilter' was
introduced; it defaults to 'False' that matches historical behaviour.

GitHub: #73
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>