Set beginning of time independently per aggregation

_From @thcrock on July 19, 2017 22:3_

from @ecsalomon: "Different data sources may have different dates at which the data become reliable (e.g., billing data may become reliable sooner than appointment data if the billing system were digitized earlier). To allows different feature aggregations to pull from different historical lengths, it would be useful to set beginning of time for each aggregation separately."

_Copied from original issue: dssg/architect#12_