Add stepCount parameter for every()

[alexmanzo]

👋 Thanks for taking the time to look at this!

This PR adds a stepCount parameter to the .every() function, which defaults to 1.

This is practical for a variety of intervals:

• biannual: every 6 months / 2 quarters
• every 2 weeks
• every 4 years
• every 90 minutes

Happy to address any feedback!

[spencermountain]

hey - this is very nicely done. thank you!