update the file at the configured manifest file path if it exists

lib/papers/manifest_command.rb

@@ -1,7 +1,7 @@
 module Papers
   class ManifestCommand
     def initialize(manifest_path = nil)
-      @manifest_path = manifest_path || File.join('config', 'papers_manifest.yml')
+      @manifest_path = manifest_path || Papers.config.manifest_file || File.join('config', 'papers_manifest.yml')
     end
 
     def manifest_exists?

spec/manifest_updater_spec.rb

@@ -170,4 +170,31 @@
       expect(updater.update).to eq(expected)
     end
   end
+
+  describe "with a configured manifest file path" do
+    let(:path) { "/config/papers_manifest_custom.yml" }
+    subject(:updater) { Papers::ManifestUpdater.new }
+    let(:original_content) { <<EOS
+#{header.chomp}
+gems:
+  rails-4.2.0:
+    license: MIT
+    license_url:
+    project_url: https://github.com/rails/rails
+EOS
+          }
+
+    it "updates the file at the configured path" do
+      Papers.configure do |config|
+        config.manifest_file = path
+      end
+
+      allow(updater).to receive(:gemspecs).and_return([
+        double(name: 'rails', version: '4.2.7.1', license: "MIT"),
+      ])
+
+      expected = original_content.gsub(/rails-4.2.0/, "rails-4.2.7.1")
+      expect(updater.update).to eq(expected)
+    end
+  end
 end