Allows overriding notification settings per alert

[marks-sortable]

This fixes issue #76, allowing individual alerts to override some notification details (e.g. email recipient, Slack channel).

Example:

      {
        "name": "site-404s",
        "time_window": "10minute",
        "method": "maximum",
        "query": "...",
        "rules": ["warning: > 10"],
        "override": {
          "slack": {
            "channel": "#site-dev"
          },
          "smtp": {
            "to": "dev-site@example.org"
          }
        }
      }

Not all fields can be overridden, nor can all alert methods. Overrides usually don't let you change which server you're talking to, or what API key you're using. Available overrides:

cli

command

hipchat

room

http

url, params, method

slack

channel, username

smtp

sender, to

victorops

url, routing_key

[coveralls]

Coverage decreased (-2.3%) to 62.777% when pulling dee22cf on marks-sortable:customize-alert-handlers into 63a9d00 on klen:develop.