Feature Request: Add ability to define custom fields to the PD handler

The customer would like the ability to define custom fields to be populated via tokenization (outside of using the -d flag)

For example:
```
check_output: "{{.Check.Output}}"
client_url: "https://sensugourl.com/c/~/n/{{.Entity.Namespace}}/events/{{.Entity.Name}}"
client: "{{.Entity.Name}}"
```
internal ref: https://secure.helpscout.net/conversation/2179913205/31023?folderId=4465878
