WIP: Use ctx when elli_ctx is set

[yurrriq]

@tsloughter, feel free to implement where I've got throw(nyi). I think I got them all... I figure we could maybe just fold in your ctx module as elli_ctx, unless you want it to stay generic.

{erl_opts, [{d, elli_ctx}]}.

^ this will trigger a bunch of failures as of now

[coveralls]

Coverage increased (+0.4%) to 76.486% when pulling 17cbcc8 on feature/abstract-elli_req into f56f2bc on develop.

[tsloughter]

Hey, sorry, looking at this again now. It seems to require putting stuff like the socket in the context?

I think we'd need to instead move to a 3 arity handle function: handle(Ctx, Req, Args)

[yurrriq]

Yeah, it does look that way.. I can't remember what I was thinking here. I do like the further adoption of #req{} accessor functions though. Seems like it'd make it easier to transition away from using a record, if we decide to use maps in the future.

I wonder if we couldn't just implement an elli_ctx_middleware module, and check handlers for handle/3 (pass context) or handle/2 (behave like elli_middleware).

I'll pull out the req accessors parts into a new PR, and can draft an elli_ctx_middleware today or tomorrow, if you like.