Add playbook/runbook link to aid operations in resolution of alerts

[pgporada]

It would be useful for an alert to be able to send back a link to some internal documentation, graphs, etc.

Alert: 1 | somehost/diskuse: ['OK - / 55% used.' 'WARNING - /opt 85% used.']
Playbook: 1 | somehost/diskuse: "You'll want to review https://example.com/link/to/internal/documentation and check historical graphs at https://example.com/grafana/graphs/located/here"

Sensu should support this https://sensuapp.org/docs/latest/reference/checks.html#example

Some example code is located at https://gist.github.com/johntdyer/aec1672ee21aede1f899

[majormoses]

Should be a pretty easy enhancement and I would be more than happy to review a PR.