Adds a new webhook integration for WordPress blogs. Both WordPress.com
and self-installed blogs are supported, with minor differences that
are described in the documentation. It creates a new message for each
action, the stream and topic may be specified or use default values.
WordPress actions supported:
publish_post: a new blog post was published
publish_page: a new page was published
user_register: a new user account was created
wp_login: a user logged in
Notes: comment_post only provides the id of the parent post, not title
or link, so was not included. On further testing, I found edit_post is
not very practical, it also fires while a new post is being written, and
when posts are deleted. (I think it tracks drafts too.) I've removed it,
as it seems more confusing than useful.
Fixes#3245
Feorlen