Report newly added CVEs/vulnerabilities

As per a discussion on gitter:

It would be nice to be able to get a diff when polling for new data. This way, when you call the vulnerablecode service, you can easily see which CVEs have been introduced since the last query. This could also look like a hook/trigger that could potentially call another service.

For example, "as a software engineer, it would be nice to be notified when a new CVE is introduced".

Below is the initial question from gitter:
`Hey! Been playing around with vulnerable code for a bit, and was wondering if there is a feature to notify when there are new CVE’s? For example, when I make an API call to my vulnerable code service, is there a way to see what info is brand new, rather than fetching all of the data or a specific ID? Essentially, I am hoping to poll my vulnerable code service and be notified if there are any new additions. Thanks!`