allow params post-processing

[iliakan]

Express had a way to post-process app.params.

For example, a param with name :id could be cast to ObjectId (and if fails, throw 404), or :userId becomes the autoloaded user.

In the particular case of using Mongo/Mongoose that was quite helpful, because if a param does not validate as ObjectId, then findById(id) and findOne(_id: id) methods throw CastError.

To handle the error, we need to either to validate it manually (in try..catch) in every route which uses user-provided id, or just rely on a global agreement (implemented by post-processing params) ":id or :...Id" are ObjectIds" could be helpful.

P.S. This functionality seems to exist for koa-route: https://github.com/segmentio/koa-params
Why not to implement it here, it fits nicely.

[alexmingoia]

See https://github.com/alexmingoia/koa-router#parameter-middleware