feature-request: support XEP-0224 (Attention)

[rigid]

For a mobile client, this feature could be useful. If the option is enabled, yaxim could override vibrating/silent mode so you get notified when people explicitly "buzz" you.

[ge0rg]

I see potential use cases for this, however there is also some abuse potential if implemented incorrectly.

I will not add this feature any time soon, but I won't object if somebody adds support.