Feature request: accept lambda rather than using uniform_notifier

[bockets]

Howdy! Wondering if you'd be open to a PR that allowed for configuration of a notifier via a lambda instead of the uniform_notifier gem. For example:

# in config/environments/development.rb

Bullet.enable = true
Bullet.on_n_plus_one do |detection|
  # do a custom action with the detected n+1
end