API endpoint to get the number of comments

Imported from https://github.com/adtac/commento/issues/78

changed the description

@adtac Btw. Glad to see you guys here! ;)

I've just deployed commento for a fleet of sites, and this is one thing that was sorely missed coming from Disqus.

mentioned in commit 330131f3

@cweagans https://gitlab.com/commento/commento-ce/commit/330131f390ca13e543050a52d46fbe715a9484b6 should add /api/comment/count :)

Example usage: curl http://commento.example.com/api/comment/count --data '{"domain": "blog.example.com", "path": "/path.html"}' (obviously, you'll write your own caller in JavaScript if you're dynamically loading Commento).

closed via merge request !43 (merged)

Awesome! I'll give this a go very soon. Thanks for tackling this!