[SPIKE] Create Content Negotiation Plug that *Any* Phoenix App can use!

[nelsonic]

We have show that content negotiation is fairly straightforward in a Phoenix App in our tutorial: https://github.com/dwyl/phoenix-content-negotiation-tutorial
but our conclusion is that it's quite cumbersome.

We need a simplified way of doing content negotiation in our App(s)
whereby we only add one line to the router.ex and one line to each controller.

I think it's possible, but I don't know for sure. 💭
I would prefer to avoid including Phoenix as a Dependency ... because of maintenance and RAM.
So this issue is a "SPIKE" to:

• explore how to do content negotiation in a reusable plug.

[nelsonic]

Didn't expect this name to be available on Hex.pm: https://hex.pm/packages/content

Going to use it.

[nelsonic]

Shipped: https://hex.pm/packages/content

Working on docs now. 📝

[nelsonic]

I have used the content plug in the tutorial "Part 2":
https://github.com/dwyl/phoenix-content-negotiation-tutorial#part-2

It works exactly how I imagined. ✅
And if I didn't have to use the & to "capture" the plugs it would look good too. 👍

[nelsonic]

done ✅