Feature request: timedeltas on dates (today-7 days, today+3 days)

As it stands, it's difficult to narrow down a drip to a specific day, for instance on the `date_joined` field. For example, If I want to send a message on a user's 6th day after joining I have to use two filters, `date_joined__gte, now-7` and `date_joined__lte, now-6`.

After this proposed patch, this can be accomplished with just one filter:
`date_joined__contains, today-6`