Add an append= keyword that works in a somewhat similar way to

Add an append= keyword that works in a somewhat similar way to
default=.  In the append= case, when looking up an entry, it also
looks in the section named by the append= for something to append
to the entry

--HG--
extra : convert_revision : b51915094ad3ca151d7f241c29e19b6b29d3a3c0