new prefer_calendar option that will display a text/calendar alternative

This commit implements a prefer_calendar option that will show the text/calendar alternative along with the text/plain or text/html multipart alternatives.

Exchange sends calendar invites as a text/calendar multipart alternative.  With this new option the ics/vcal info is now seen at the end of the message and can be piped to an application to handle the invite.  I use the follow bindings in thread mode for importing calendar invites to Google Calendar:

```
[bindings]
    [[thread]]
        c v = pipeto --format decoded --shell 'sed /BEGIN:VCALENDAR/,/END:VCALENDAR/\!d | gcalcli import -d | less'
        c V = pipeto --format decoded --shell 'sed /BEGIN:VCALENDAR/,/END:VCALENDAR/\!d | gcalcli --reminder=10 --detail_url=short --calendar="Eric Davis" import | less'
```