Subscription based payment model

[matthew-shaw]

Is your feature request related to a problem? Please describe.
Introduce a subscription based payment model with various tiers of pricing reflective of increasing levels of functionality and data retention.

Describe the solution you'd like
Maintain a free entry-level tier, with minimal but usable levels of functionality and data retention. Introduce 2-3 additional levels of functionality above the free tier, with a non-linear sliding scale of price, such that the top tier costs the most, but also represents the greatest value proposition.

Describe alternatives you've considered
The product should remain entirely free to use for as long as possible. But there may come a time when it will need to be self sufficient and request users pay for different levels of functionality to fund itself.