Add linter field to JSON reporter

This change adds a new field `linter` into `offenses` with the JSON reporter.
The default reporter outputs linter names,
so I think it useful if the JSON reporter also does it.

.gitignore

@@ -1,3 +1,4 @@
 Gemfile.lock
 /coverage
 /pkg
+.ruby-version

lib/slim_lint/reporter/json_reporter.rb

@@ -45,6 +45,7 @@ def map_offense(offense)
         location: {
           line: offense.line,
         },
+        linter: offense.linter&.name,
       }
     end
   end

spec/slim_lint/reporter/json_reporter_spec.rb

@@ -48,10 +48,12 @@
       let(:lines)        { [502, 724] }
       let(:descriptions) { ['Description of lint 1', 'Description of lint 2'] }
       let(:severities)   { [:warning, :error] }
+      let(:linters) { [double(name: 'SomeLinter'), nil] }
 
       let(:lints) do
         filenames.each_with_index.map do |filename, index|
-          SlimLint::Lint.new(nil, filename, lines[index], descriptions[index], severities[index])
+          SlimLint::Lint.new(linters[index], filename, lines[index], descriptions[index],
+                             severities[index])
         end
       end
 
@@ -60,6 +62,16 @@
         output['files'].map { |f| f['path'] }.sort.should eq filenames.sort
       end
 
+      it 'list of offenses' do
+        subject
+        output['files'].sort_by { |f| f['path'] }.map { |f| f['offenses'] }.should eq [
+          [{ 'linter' => nil, 'location' => { 'line' => 724 },
+             'message' => 'Description of lint 2', 'severity' => 'error' }],
+          [{ 'linter' => 'SomeLinter', 'location' => { 'line' => 502 },
+             'message' => 'Description of lint 1', 'severity' => 'warning' }],
+        ]
+      end
+
       it_behaves_like 'output format specification'
     end
   end