Add step_time_before() and step_time_after()

[EmilHvitfeldt]

These steps would answer the question such as

• "how many days since Sunday"
• "Number of days before Christmas"

The reference point should be able to be a vector of date-times or a recurrent event generated with almanac (hopefully, haven't prototyped yet)

There should additionally be a decay argument to modify how "n periods before x" are returned. Exponential and logarithmic are prime candidates

Maybe a step_time_period() which I see as a combination of before + after

[EmilHvitfeldt]

Add unit argument that specifies how the number is calculated, days, weeks, months. Etc etc