[WIP] Explicit proxies

Just a WIP for having more complex proxy behavior.

Proposed changes:

  - [x] renames `api-stub` generator to `http-mock`, which is maybe more accurate? :bikeshed:
  - [x] adds an `http-proxy` with example proxy: `ember g proxy api 127.0.0.1:8000`
  - [x] updates `server` directory to include `mocks` and `proxies`
  - [x] makes proxies happen last in mocks, middlewares, proxies ordering
  - [x] make proxies with with requests that need to be streams
  - [x] tests

Nice to have
  - [ ] leaves `--proxy` behavior, but warn if used when explicit  proxies are generated