Feature Request: Default alert details should be configurable

For example, an alert will usually always contain Influx tags, so I might want to set my default alert message to this:

```

{{ range $key, $value := .Tags }}

{{ end }}

Tags
{{ $key }}
{{ $value }}

```

Right now, there's not a convenient way to do this without duplicating the information in multiple TICKscripts (including in templates).

If there was a global setting, I could point to a file containing my details, e.g.:

```
[deadman]
details = file:///etc/kapacitor/details.txt
```

No source file, test, or entry point is named. Start by tracing how alert details, TICKscripts, templates, and global configuration are currently handled. Done means a global setting can reference a details file such as /etc/kapacitor/details.txt and make its contents available in default alert messages without duplicating them across TICKscripts.

Written by the indexing model from the issue text.