Parse and Extract durations / spans of time

[akoumjian]

We want to be able to extract durations or spans of time. For example "20 days". In Python's standard library this is represented as a timedelta.

It's not clear yet if this is compatible with the find_dates method and may need to be employed as a separate method. For example, this would likely cause complications with a different feature request #170 as "20 days ago" describes a relative date, not a timedelta.