Support programmatic usage of @decent/server better

[bates64]

Currently, @decent/cli has to directly edit the database of the running server when commands are run. It'd be nice to allow require('@decent/server') consumers to pass some kind of object in with config and for it to export a nice programmatic API for them.

Thoughts?

[towerofnix]

This would be nice, but probably hard to implement. We currently seriously rely on Express's middleware system, both for behavioral logic and dealing with errors/bad input. I'm not sure how to fake Express middleware accurately or how to fit this into our existing code nicely. I have an "evaluate middleware" function I made for the server tests, but it's certainly not an exact clone of how Express works; it's just there so we can test each middleware function as its own unit.

[bates64]

@towerofnix it would be fun to abstract the API more - eg. user.create function which is used by POST /api/users would be used both in the express code and also exported by @decent/server.