Add default config in `config.exs` + ability to configure Tesla middleware

[ggiill]

Inspired by #42

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Adding the ability to configure default options in config.exs:

Configure default settings in config/config.exs (optional).

# config/config.exs
config :ex_force, :api_version, "43.0"
config :ex_force, :adapter, {Tesla.Adapter.Hackney, [recv_timeout: 1_000]}

As well as the ability to pass in custom Tesla Middleware:

# config/config.exs
config :ex_force, :middleware, [
   Tesla.Middleware.Telemetry,
   {Tesla.Middleware.Timeout, timeout: :timer.seconds(1)}
 ]