emacs: Allow renderer of multipart/related parts access to non-primary parts.

emacs: Allow renderer of multipart/related parts access to non-primary parts.

Typically used to allow a `text/html' renderer access to images which
are sent along with the HTML.

This is not enabled by default, instead the user must execute
`notmuch-show-setup-w3m' for it to take effect.

Edited-by: Carl Worth <cworth@cworth.org> Add documentation string for
notmuch-show-setup-23m and clean up warning about reference/assignment
of free variable.

Signed-off-by: Jameson Rollins <jrollins@finestructure.net>