Added configuration loader

Necessary to load the configuration with preference for any defaults set via the environment.

lib/trmnl/api/configuration/loader.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module TRMNL
+  module API
+    module Configuration
+      # Loads configuration based on environment or falls back to defaults.
+      class Loader
+        def initialize model: Content, environment: ENV
+          @model = model
+          @environment = environment
+        end
+
+        def call
+          model[
+            content_type: environment.fetch("TRMNL_API_CONTENT_TYPE", "application/json"),
+            uri: environment.fetch("TRMNL_API_URI", "https://trmnl.app/api")
+          ]
+        end
+
+        private
+
+        attr_reader :model, :environment
+      end
+    end
+  end
+end

spec/lib/trmnl/api/configuration/loader_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe TRMNL::API::Configuration::Loader do
+  subject(:loader) { described_class.new environment: Hash.new }
+
+  describe "#call" do
+    it "answers default configuration when environment is unset" do
+      expect(loader.call).to eq(
+        TRMNL::API::Configuration::Content[
+          content_type: "application/json",
+          uri: "https://trmnl.app/api"
+        ]
+      )
+    end
+
+    it "answers custom configuration when environment is set" do
+      loader = described_class.new environment: {
+        "TRMNL_API_CONTENT_TYPE" => "application/xml",
+        "TRMNL_API_URI" => "https://api.trmnl.com"
+      }
+
+      expect(loader.call).to eq(
+        TRMNL::API::Configuration::Content[
+          content_type: "application/xml",
+          uri: "https://api.trmnl.com"
+        ]
+      )
+    end
+  end
+end