cli: add --body=true|false option to notmuch-show.c

cli: add --body=true|false option to notmuch-show.c

This option allows the caller to suppress the output of the bodies of
the messages. Currently this is only implemented for format=json.

This is used by notmuch-pick.el (although not needed) because it gives
a speed-up of at least a factor of a two (and in some cases a speed up
of more than a factor of 8); moreover it reduces the memory usage in
emacs hugely.