Refactor to base library

[deadtrickster]

The idea is to have this lib as a foundation for other exporters - plugs, elli, cowboy, etc.

prometheus_httpd_config stays the same.
prometheus_http will have shared /metrics behaviour with two exports:

render/1

 `render(Env)`

Env is a map:

%{
  path => Path. %% present if you want prometheus_http to handle routing and registry extraction
  accept => Accept, %% accepted content types
  accept_encoding => AcceptEncoding, %% accepted encodings
  registry => Registry, %% ignored if path present, can generate 409 if registry config isn't auto
}

index/0

Renders standard welcome page with useful links and current metrics path.

Both return {Code, Headers, Body}.

[deadtrickster]

I think this is exactly what prometheus_http_impl does, And it's used by a couple of libs already.