doc: add default doc:get-attr which returns pane attribute.

doc: add default doc:get-attr which returns pane attribute.

Sometimes we want an attribute which can vary with location in the document,
but offen i satisfied with a document-wide value.
fill:start-re is a good example.

Provide a default doc:get-attr, which will be called if no attr can be
found at the specific location.  This returns a pane attribute is
possible.

Also ensure all current doc:get-attr function fall through if they
cannot find a value.

Signed-off-by: NeilBrown <neil@brown.name>