keen.io: fixing addons without input

It turns out the Keen.io API will complain if we pass it addons without any sort of input. 

> {"name":"InvalidAddonError","description":"Your event contained an invalid add-on. IP to Geo Enricher's 'ip' points to 'ip_address' but that property does not exist in the event you sent!"}

this commit fixes that so we'll only include the addons if it's both enabled and the field is there

@yields @gjohnson
i'm go back and forth on having this extra file. kind of nice to split it up in case keen adds more add-ons, but I don't want most integrations having a bunch of files floating around.