[RCD] new hook request. allow plugins to control inline message parts

Hi,

RC helpfully displays attached emails inline under the primary message. This is handy when people forward you emails as attachments. IMO it is not so handy with spam reports from SpamAssassin. These reports contain a summary of the message so I do not need to look at the original unless I really want to.

My idea is to add a plugin hook into the rcmail_message_body function to allow plugins to prevent a message part from being displayed. This is similar to the message_part_get hook which lets plugins control attachment view/download.

Phil