Add support to access an advisory's comments

[johnbieren]

We should add support to use the API to get an advisory's comments.
The API endpoint is just comments but that is kind of useless. I suggest we add the endpoint
"comments?filter[errata_id]=${advisory_id}" so we can get all comments for a certain advisory. It is possible to use further filters, but the ability to get comments for a given advisory would be a great first step