Add index-format-hook and expando

Add index-format-hook and expando

index-format-hook is used to allow dynamic insertion/evaluation of
format strings into $index_format.

It can be used, for example, to implement date formatting based on the
age of the message.

Add a new %@name@ expando to $index_format, which evaluates the
matching index-format-hooks with "name".

Co-authored-by: Richard Russon <rich@flatcap.org>