[PATCH v5] Make buttons for attachments allow viewing as well as saving

[Austin Clements]

LGTM

Quoth Mark Walters on Jan 20 at  9:44 am:
> Define a keymap for attachment buttons to allow multiple actions.
> Define 3 possible actions:
>     save attachment: exactly as currently,
>     view attachment: uses mailcap entry,
>     view attachment with user chosen program
> 
> Keymap on a button is: s for save, v for view and o for view with
> other program. Default (i.e. enter or mouse button) is save but this
> is configurable in notmuch customize.
> 
> One implementation detail: the view attachment function forces all
> attachments to be "displayed" using mailcap even if emacs could
> display them itself. Thus, for example, text/html appears in a browser
> and text/plain asks whether to save (on a standard debian setup)