add part_sep formatter to replace "first" argument to part format functions

add part_sep formatter to replace "first" argument to part format functions

A new field "part_sep" is added to the notmuch_show_format structure,
to be used for part separation.  This is cleaner than the "first"
argument that was being passed around to the part arguments, and
allows the function that handles overall part output formatting
(show_message_part) to directly handle when outputting the separator.