Directories can now have default view types associated with them, by
the simple yet sleazy method of touching a file in them called
_.flag.prefview:~~<viewname>~~_.

This lets us auto-set a directory to default to a blogdir view.

The page tools now autogenerate a list of 'See As <whatever>' links
for alternate directory formats.

This required some innards magic to distinguish a normal view being
explicitly specified from no view being specified and us just
defaulting to a normal view.