no issue
- NQL does not support the relationship setup that members->stripe customer<->stripe subscriptions uses so it wasn't possible to use the `filter` param to query against having an active subscription
- adds `customQuery` bookshelf plugin that allows customisation of SQL query used in `findPage` method by individual models
- use `customQuery` in Member model to set up joins and conditionals to select free/paid members when `options.paid` is present
- allow `?paid` param through API and permitted options for member model
