Commerce stepped proportional pro-rata

This module provides a pro-rata plugin for Commerce Recurring which allows
proportional pro-rata prices at specific intervals in the billing period, and a billing schedule plugin with allows for a free rollover interval at the end of a fixed period.

For example, with a billing period of 1 year, and 4 steps, a price of 100 would
be charged at:

• 0-3 months: 25
• 3-6 months: 50
• 6-9 months: 75
• 9-12 months: 100

With a rollover period of 1 month, purchasing a subscription in the final month of the fixed period would be charged at the full price, but also give you the following billing period.