Add specs for Options

This brings the coverage in this file up to 100%.

lib/slim_lint/options.rb

@@ -44,10 +44,22 @@ def add_linter_options(parser)
 
       parser.on('-r', '--reporter reporter', String,
                 'Specify which reporter you want to use to generate the output') do |reporter|
-        @options[:reporter] = SlimLint::Reporter.const_get("#{reporter.capitalize}Reporter")
+        @options[:reporter] = load_reporter_class(reporter.capitalize)
       end
     end
 
+    # Returns the class of the specified Reporter.
+    #
+    # @param reporter_name [String]
+    # @raise [SlimLint::Exceptions::InvalidCLIOption] if reporter doesn't exist
+    # @return [Class]
+    def load_reporter_class(reporter_name)
+      SlimLint::Reporter.const_get("#{reporter_name}Reporter")
+    rescue NameError
+      raise SlimLint::Exceptions::InvalidCLIOption,
+            "#{reporter_name}Reporter does not exist"
+    end
+
     # Register file-related flags.
     def add_file_options(parser)
       parser.on('-c', '--config config-file', String,

spec/slim_lint/options_spec.rb

@@ -39,6 +39,32 @@
       end
     end
 
+    context 'with a reporter option' do
+      context 'for a reporter that exists' do
+        let(:args) { %w[--reporter Json] }
+
+        it 'sets the `reporter` option' do
+          subject.should include reporter: SlimLint::Reporter::JsonReporter
+        end
+      end
+
+      context 'for a reporter that exists when capitalized' do
+        let(:args) { %w[--reporter json] }
+
+        it 'sets the `reporter` option' do
+          subject.should include reporter: SlimLint::Reporter::JsonReporter
+        end
+      end
+
+      context 'for a reporter that does not exist' do
+        let(:args) { %w[--reporter NonExistent] }
+
+        it 'raises an error' do
+          expect { subject }.to raise_error SlimLint::Exceptions::InvalidCLIOption
+        end
+      end
+    end
+
     context 'with the help option' do
       let(:args) { ['--help'] }