Allow configuration of build notifications to be made from gitlab-ci.yml

I'd like to be able to, from within the gitlab-ci.yml configuration, specify notification channels to indicate build status'.

For example, the following configuration could send messages to the 'slack' service and will send notifications to a number of rooms and users filtered by state.

It can deal with having multiple configurations based on different scenarios, and could also be tied to branches in the same way jobs are.

notifications:
  slack-failures:
     service: slack
     channels: [ "#aroom", "#anotherroom", "@user" ]
     states: ['ok', 'failure']
  slack-all:
     service: slack
     channels: [ "#buildevents" ]
     only: # As per jobs - only notify on master builds 
       - master