Reconsider the criteria for personal emails

[mardukbp]

Currently mu index considers that emails such that one of the recipients is my email address are personal. It is easy to see that this condition does not define the set of personal messages, see #148. I think that a better definition would be the current one augmented with the condition that the only recipient is my email address. Now that is really personal and results in a clean and useful list of addresses for auto-completion. I am not saying this should be the default behavior of mu index, but it should definitely be a command line option --personal.

[djcb]

You have a point, but the 'personal' concept is mostly there to give a bit of guidance to address completion in mu4e; and with the latest changes there, it seems good enough. So, I'm not really interested in complicating the code for that - though indeed, there's room for all kind of customization.