CB Paid Subscriptions

This integration is well equipped to modify plan prices easily. Implement support for adjusting the base price of a plan dynamically. For example support substitutions (e.g. [cb_quotedprice]) and modifiers (e.g. +10, -15, *3, +[cb_priceadjust]). This avoids needing to use promotions to alter the base pricing of plans and will also work better with pricing options since the base price is adjusted instead of the totalized price.