Support additional PD-CEF fields and optionally send templated details as Struct/JSON

[atomicules]

This is a single issue, but contains two parts:

1. A request to add support for the remaining PD-CEF fields of:
   1. Timestamp
   2. Class
   3. Group
2. A request to support processing the details template as json so we can send structured data through to Pagerduty. At the moment the options are:
   1. Send through the full event JSON. This is a lot of unnecessary data and the deep nesting of fields (when using labels and annotations) makes it hard to read some fields on Pagerduty
   2. Use a template to select required fields. Which unfortunately sends through the data as a string with no structure on the Pagerduty side which means you can't really use custom details in event rules

I'm hoping 1 is pretty uncontroversial and can be done. I would prefer it if these work similarly to summary and use a template.

I could accept 2 not happening if 1 goes ahead, but it really would be nice to have the option of templating structured data one-way or another.

In accordance with the contributing guidelines I'm opening an issue to discuss first, but I have made initial code changes on this branch. The changes can be seen on this compare view

I have tried to separate the commits to cover both requests:

• These two commits for adding the additional PD-CEF fields.
• This commit which adds a --send-details-as-json flag to process the template as json.

There is undoubtably a better way of doing 2 than what I've done - and I definitely welcome feedback and guidance. What I've done is pretty simplistic: When I realised that using the template returned a string I just used a json.Unmarshal to parse that string as a Struct/JSON. It "works", but maybe a real solution lies deeper in the templating stuff?