[Discussion] Move Akismet site and apiKey properties to siteConfig?

I'm looking to make staticman working with multiple site but the Akismet configuration is confusing me.
Although they were mentioned before (#83, #339), whether it should be parts of `staticman.yml` is not clear. There are 2 opinions exist:
1. `site` should be the url where staticman instance hosted.
2. `site` should be the url of gh-pages where guest posting comments on.

I suggest moving those 2 properties to siteConfig for following reason:
* By forwarding but not owning `apiKey`, a single instance can process as many site as possible.
* According those material, `site` is parts of Akismet's price calculation.
* [What’s an API key?](https://docs.akismet.com/getting-started/api-key/)
* [Should I choose a free or paid subscription?](https://docs.akismet.com/getting-started/free-or-paid/)
* [Adding Sites to an Existing Plus Subscription](https://docs.akismet.com/account/adding-sites-to-an-existing-plus-subscription/)
* Error codes [10007](https://akismet.com/errors/10007/) and [10008](https://akismet.com/errors/10008/)
* [API docs](https://akismet.com/development/api/#comment-check):
```
### PARAMETERS
`blog` (required)
The front page or home URL of the instance making the request. For a blog or wiki this would be the front page. Note: Must be a full URI, including http://.
```

Discussions are welcome.

Review discussions #83 and #339 and the linked Akismet API material first. Clarify whether site and apiKey belong in staticman.yml or siteConfig, then verify that the chosen configuration supports one Staticman instance processing multiple sites and makes the ownership of each value clear.

Written by the indexing model from the issue text.