Include example template + check hook output

[asachs01]

This solves for #27 and #30 by providing a workaround in the form of a template + check hook output that can be put into the template body until the plugin is refactored so that hook output and templates aren't mutually exclusive.

[nixwiz]

Still needs some format changes. How about this?

Greetings,<br>
<br>
<h3>Informational Details</h3>
<b>Check</b>: {{ .Check.Name }}<br>
<b>Entity</b>: {{ .Entity.Name }}<br>
<b>State</b>: {{ .Check.State }}<br>
<b>Occurrences</b>: {{ .Check.Occurrences }}<br>
<b>Playbook</b>: https://example.com/monitoring/wiki/playbook<br>
<h3>Check Output Details</h3>
<b>Check Output</b>: {{.Check.Output}}<br>
<h4>Check Hook(s)</h4>
{{range .Check.Hooks}}<b>Hook Name</b>:  {{.Name}}<br>
<b>Hook Command</b>:  {{.Command}}<br>
<b>Hook Output</b>: {{.Output}}<br>
{{end}}

<br>
#monitoringlove,<br>
<br>
Sensu<br>
</html>

However, this does bring to light one issue. When using HTML, the formatting of the hook output (or any multi-line string) won't be properly formatted. Here's an example using the template above.

[asachs01]

@cwjohnston can I get a 👍 on this?

[cwjohnston]

Suggested some manifest -> definition changes.

Also, it seems like this handler has some command line flags that should be covered in a usage section.

[asachs01]

@cwjohnston can you take a gander again plz?