outlook message/diary handling

[Nic Ferrier]

We have function diary-from-outlook and the diary-outlook-formats alist
to help integrate with MS-Outlook appointment madness.

I've got an additional format for that which I'll try and submit but I
think the interface could do with changing.

1. Right now diary-from-outlook is fixed to two email implementations,
rmail and gnus. But we have lots of other mailers that would benefit
from the same functionality.

2. diary-from-outlook ends with diary-from-outlook-internal which only
handles diary entries in Emacs' diary format. But many people like to
keep their diary in org mode. So being able to add entries in either
format would be good.

I'd be happy to provide some improvement here if people agree.