tell: use config to send reminders as private messages

[Exirel]

This fix #1151

• config.tell.use_private_reminder (y/n) [n]: send tell/ask reminders as private messages
• config.tell.maximum_public [4]: maximum number of tell/ask reminders to send as public messages before sending them as private messages

As a bonus point: minor refactoring with unit tests.

[dgw]

This looks 👍 in general, but uh… Why bring io into this? The only thing I see used from that module, io.open(), is just an alias to the built-in open().

[Exirel]

Why bring io into this? The only thing I see used from that module, io.open(), is just an alias to the built-in open().

Because Python 2.7's open function doesn't know about "encoding". Since I wanted to deal with unicode string only, I switched to io.open.

[dgw]

Fair enough about io. Now I just hope this doesn't cause conflicts with #1699. 😸

[Exirel]

Now I just hope this doesn't cause conflicts with #1699.

I think it'll. But that's okay, the change aren't that big, and you now have unit-tests to help you!