feature request: monthly membership model?

[spencersevilla]

One of the communities outside Oaxaca apparently uses a monthly membership model, and I think it'd be a good feature to add to haulage. Generally speaking I don't want to make haulage too feature-heavy, would prefer integrate with existing WISP management tools than race them. Compared to these tools, the biggest value-adds haulage provides are (1) it supports prepaid customers (2) its simple and (3) its free. I think/hope we could implement this with minimal logic - a database field to mark whether the user is on a monthly or data plan, and then some logic to automatically draw-down from their $ balance every month, mark how long they're paid through, and cut them off if they ever go below 0.

[matt9j]

I can implement this once PR #17 lands. I don't want to make too large of a diff before then though.