Adds a setting to the configuration to configure at which level the
folders in the dirtree are collapsed by default.
In my case, this is useful because my organisation has some rather deep
nesting in the folder structure, and a _lot_ of folders, and this way I
can keep my dirtree uncluttered while still having all folders there if
I need them.
Signed-off-by: Sijmen <me@sijman.nl>