feat(formatter): add "except" and "only" rubocop formatter options

They take a list or comma separated string of cops, which will be passed
to rubocop as --except and/or --only.

The "except" option is useful to disable cops which can cause annoyance
when using solargraph's formatter as a before save hook in text
editors.

I'm less aware of what the "only" option might be useful for, but might
as well add it so both are covered.

lib/solargraph/language_server/message/text_document/formatting.rb

@@ -41,14 +41,26 @@ def config_for(file_uri)
           end
 
           def cli_args file, config
-            [
+            args = [
               config['cops'] == 'all' ? '--auto-correct-all' : '--auto-correct',
               '--cache', 'false',
               '--format', 'Solargraph::LanguageServer::Message::' \
                           'TextDocument::Formatting::BlankRubocopFormatter',
-              config['extra_args'],
-              file
-            ].flatten
+            ]
+
+            ['except', 'only'].each do |arg|
+              cops = cop_list(config[arg])
+              args += ["--#{arg}", cops] if cops
+            end
+
+            args += config['extra_args'] if config['extra_args']
+            args + [file]
+          end
+
+          def cop_list(value)
+            value = value.join(',') if value.respond_to?(:join)
+            return nil if value == '' || !value.is_a?(String)
+            value
           end
 
           # @param original [String]

lib/solargraph/workspace/config.rb

@@ -154,6 +154,8 @@ def default_config
           'formatter' => {
             'rubocop' => {
               'cops' => 'safe',
+              'except' => [],
+              'only' => [],
               'extra_args' =>[]
             }
           },