Add CommentOutputHook

[raviks789]

CommentOutputHook gets the transformation for the comments. For example creating ticket links in the comments using icinga web generic tts module.

[nilmerg]

Comparing this with the PluginOutputHook, the latter also gets two additional parameters passed: $commandName and $enrichOutput.

I think we should pass similar parameters here. There's no command name of course, but an author's name.

Also, $enrichOutput is meant to control whether HTML is processed/generated or not. A similar thing is possible here since Markdown is supported. Though, we always render Markdown, just not all types of it. Lists for example only render inline elements. Block elements are only rendered in the detail views. This differentiation should also be possible for hooks, so let's pass them the $viewType. ('list' or 'detail', use constants for these, don't just pass hardcoded strings)

[nilmerg]

Said all this, we're not gonna merge this anytime soon. (But please finish this as I described above) I've talked about this with @lippserd and we agreed to postpone this to next year Q2. The reason for this is that we're not sure yet if the interface should be kept this way or if it should allow passing the comment model as well. This may allow implementors to retrieve more meta data (such as groups and custom variables) to generate more sophisticated details.