Add rule's name to flycheck formatter

spec/ameba/formatter/flycheck_formatter_spec.cr

@@ -21,7 +21,9 @@ module Ameba::Formatter
         s.error DummyRule.new, 1, 2, "message"
         subject = flycheck
         subject.source_finished s
-        subject.output.to_s.should eq "source.cr:1:2: E: message\n"
+        subject.output.to_s.should eq(
+          "source.cr:1:2: E: [#{DummyRule.name}] message\n"
+        )
       end
 
       it "properly reports multi-line message" do
@@ -29,7 +31,9 @@ module Ameba::Formatter
         s.error DummyRule.new, 1, 2, "multi\nline"
         subject = flycheck
         subject.source_finished s
-        subject.output.to_s.should eq "source.cr:1:2: E: multi line\n"
+        subject.output.to_s.should eq(
+          "source.cr:1:2: E: [#{DummyRule.name}] multi line\n"
+        )
       end
 
       it "reports nothing if location was not set" do

src/ameba/formatter/flycheck_formatter.cr

@@ -4,9 +4,9 @@ module Ameba::Formatter
       source.errors.each do |e|
         next if e.disabled?
         if loc = e.location
-          output.printf "%s:%d:%d: %s: %s\n",
+          output.printf "%s:%d:%d: %s: [%s] %s\n",
             source.path, loc.line_number, loc.column_number, "E",
-            e.message.gsub("\n", " ")
+            e.rule.name, e.message.gsub("\n", " ")
         end
       end
     end