timesince 'depth' parameter

[Toby Such]

Discussed ​here. The timesince function is a bit limiting as it is right now. A depth parameter should be added to configure how many values are shown. The depth parameter should be defaulted to 2 as this is how the current implementation behaves. The existing rule of values having to be adjacent to one another should still remain.

The logic for calculating the time since, before formatting should also be pulled out of the function and placed in its own so that custom implementations can be created.

For example:
With a depth of one it should always display as "1 week" or "3 years" etc.
With a depth of two: "1 week, 3 days" or "3 years, 7 months"
With a depth of three: "1 week, 3 days, 5 hours" or "3 years, 7 months, 2 weeks"

[Claude Paroz]

As discussed on the ML, the most important thing is to make it easily customizable.