Added DISCUSSION_SPAM_CHECK trigger

[ClayDowling]

Added the trigger and performed a small refactor to put all of the spam
checking logic into a single method. A note was also added to the
README concerning use of the trigger.

The intent of this pull request is to add a trigger for interaction with the StopForumSpam plugin. The trigger is used by ClayDowling/StopForumSpam#4 and there is a demo running at http://claydowling.com

[Klap-in]

Could you please rename it to something that is not a double negative?
(_not_spam()===false is just isSpam(), it isn't?)

[michitux]

In general I think this is a very good idea. However, there are at least two issues to solve before merging this:

• The event should be prefixed with PLUGIN_, see also emmited events should be prefixed michitux/dokuwiki-plugin-move#1
• Instead of passing the internal comment array it would be good to pass an explicit structure with all fields that are (potentially) relevant for spam checking.

Some more thoughts:

• The word block check could be the default action such that it can also be disabled by a plugin handling the event.
• Being able to pass a custom message for the user might be a good idea.
• Instead of not saving the message a possible action could also be to save the comment as hidden such that it needs to be explicitly approved by a moderator.