Add support for getting slices of days

[hathix]

We'd like to get the last 24 hours of data (#351 and #385) and, more generally, get data for partial days. I don't think we can do that right now -- we can only make GraphQL queries for whole days. Let's add support for that...

[exxonvaldez]

Note that we can do this now with GraphQL, but it would only be the stop to stop stats. Not the route summaries which depend on the precomputed wait and trip times.

https://github.com/trynmaps/metrics-mvp/blob/master/docs/metrics_api.md

I think this issue should be referring to the precomputed wait and trip time files. We need to be able to fetch combinations of several time intervals from one day with different time intervals from the next day.