Add support for limiting the serie to a certain time frame

[Bert Hajee]

Currently, when querying the rollup, there is no support to limit the output to a specific time frame.

I'd like to be able to, optionally, limit the returned series to a specific time frame.

Rollup.series("New Users", interval: 'hour', time_frame: Date.today.weeks_ago(2)..Date.today)