Configuration templates for custom collectors/exporters.

When useed, generates accessor for each configuration option:

iex(4)> defmodule MyInstrumenter do
...(4)>   use Prometheus.Config, [:required_option,
...(4)>                           registry: :default]
...(4)> end
iex(5)> MyInstrumenter.Config.registry(MyInstrumenter)
:default
iex(6)> MyInstrumenter.Config.required_option!(MyInstrumenter)
** (Prometheus.Config.KeyNotFoundError) mandatory option :required_option not found in PrometheusTest.MyInstrumenter instrumenter/collector config
iex(7)> Application.put_env(:prometheus, MyInstrumenter,
...(7)>                     [required_option: "Hello world!"])
:ok
iex(8)> MyInstrumenter.Config.required_option!(MyInstrumenter)
"Hello world!"