Fixed the filtering based on N[d|w|m|y] from/to a given date.
Now you can go backwards/forward as many days/weeks/months/years, from
a given date or to a given date, as you like.
Examples:
- 2017-07-01+3d: return data from the 2017-07-01 to 2017-07-04
- 2017-07-01-1w: return data from 2017-06-26 to 2017-07-01
The -N[d|w|m|y] filter works pretty much like what we already had for
gathering data from N days/weeks/months/years ago from the current
date.