Refactor shell colors

To not use a String to constantize anymore.

lib/alchemy/shell.rb

@@ -6,6 +6,13 @@ module Alchemy
   # in a list on the shell / log
   #
   module Shell
+    COLORS = {
+      clear: Thor::Shell::Color::CLEAR,
+      green: Thor::Shell::Color::GREEN,
+      red: Thor::Shell::Color::RED,
+      yellow: Thor::Shell::Color::YELLOW
+    }.freeze
+
     def self.silence!
       @silenced = true
     end
@@ -97,12 +104,7 @@ def log(message, type = nil)
     # @return [String]
     #
     def color(name)
-      color_const = name.to_s.upcase
-      if Thor::Shell::Color.const_defined?(color_const)
-        "Thor::Shell::Color::#{color_const}".constantize
-      else
-        ""
-      end
+      COLORS[name]
     end
   end
 end

spec/libraries/shell_spec.rb

@@ -99,23 +99,14 @@ class MyToDoList
 
     describe '.color' do
       context 'if given name is a constant of Thor::Shell::Color' do
-        before do
-          allow(Thor::Shell::Color).to receive(:const_defined?).and_return(true)
-        end
-
         it "should call the constant" do
-          expect_any_instance_of(String).to receive(:constantize).and_return('')
-          MyToDoList.send(:color, :red)
+          expect(MyToDoList.send(:color, :red)).to eq(Thor::Shell::Color::RED)
         end
       end
 
       context 'if given name is not a defined constant of Thor::Shell::Color' do
-        before do
-          allow(Thor::Shell::Color).to receive(:const_defined?).and_return(false)
-        end
-
-        it "should return en empty string" do
-          expect(MyToDoList.send(:color, :not_existing)).to eq('')
+        it "should return nil" do
+          expect(MyToDoList.send(:color, :not_existing)).to be_nil
         end
       end
     end