make sure parsing stops after the first ascii armor ending delimiter

[Valodim]

This is a slight improvement for handling autocrypt setup messages. See #267 for a discussion

[dkg]

This looks like a sensible resolution for #267 to me, because it means it's possible to stuff additional keys after the first ASCII-armored block.

[r10s]

Looks good to me.

Should we mention explicitly that stuff before the first ascii armor starting delimiter must also be ignored?
Just in case we decide to add sth. before later. Or do we switch to v2 then?

[dkg]

Right now it says that the cleartext MUST start with the ASCII-armor starting delimiter. I think that's a perfectly fine constraint, and i wouldn't want to relax it.