Add event hooks

[pilcrowonpaper]

Add an interface to hook into internal events for logging and analytics. This shouldn't be used for running db queries that always need to succeed.

I'm unsure if it should return error.

type EventHandlerInterface interface {
  HandleEvent(timestamp time.Time, event string, stringTags map[string]string, intTags map[string]int64, boolTags map[string]int64) error
}

Some example events are:

• signup:created
• signup:email_address_verification_code_sent
• signup:email_address_verified
• signup:password_set
• signup:completed

It would be nicer to have a method for each event but I'm not sure how that would work if you only wanted to listen to a subset of events.