Mia API images

A few things to think about:

- [ ] How to handle adapter dependencies? `mia-api` needs redis, but should that go into `package.json` if nobody else will be using our adapter?
- [ ] should adapters be responsible for downloading and tiling the image? Both [`directory` L9-L16](https://github.com/artsmia/tilesaw/blob/b8a2938f370e0257ce3ed7cd3d8a9ad48b6b1786/adapters/directory.js#L9-L16) and [`mia-api` L26-L35](https://github.com/artsmia/tilesaw/blob/b8a2938f370e0257ce3ed7cd3d8a9ad48b6b1786/adapters/mia-api.js#L26-L35) do the same thing