Allow to configure formatter via configuration file

closes #20

spec/ameba/cli/cmd_spec.cr

@@ -3,10 +3,6 @@ require "../../../src/ameba/cli/cmd"
 
 module Ameba::Cli
   describe "Cmd" do
-    it "has a list of available formatters" do
-      AVAILABLE_FORMATTERS.should_not be_nil
-    end
-
     describe ".run" do
       it "runs ameba" do
         r = Cli.run %w(-f silent file.cr)
@@ -58,7 +54,7 @@ module Ameba::Cli
 
       it "allows args to be blank" do
         c = Cli.parse_args [] of String
-        c.formatter.should eq :progress
+        c.formatter.should be_nil
         c.files.should be_nil
         c.only.should be_nil
         c.except.should be_nil

spec/ameba/config_spec.cr

@@ -4,6 +4,10 @@ module Ameba
   describe Config do
     config_sample = "config/ameba.yml"
 
+    it "should have a list of available formatters" do
+      Config::AVAILABLE_FORMATTERS.should_not be_nil
+    end
+
     describe ".load" do
       it "loads custom config" do
         config = Config.load config_sample
@@ -45,6 +49,17 @@ module Ameba
         config.formatter = formatter
         config.formatter.should eq formatter
       end
+
+      it "allows to set formatter using a name" do
+        config.formatter = :progress
+        config.formatter.should_not be_nil
+      end
+
+      it "raises an error if not available formatter is set" do
+        expect_raises(Exception) do
+          config.formatter = :no_such_formatter
+        end
+      end
     end
 
     describe "#update_rule" do