Respect the use of ENV['NO_COLOR']

Respect the declaration of the of the NO_COLOR environment variable as a sign
that color output should be suppressed in he tool. This informal
standard is described at https://no-color.org

lib/thor/shell/color.rb

@@ -97,7 +97,11 @@ def set_color(string, *colors)
     protected
 
       def can_display_colors?
-        stdout.tty?
+        stdout.tty? && !are_colors_disabled?
+      end
+
+      def are_colors_disabled?
+        !ENV['NO_COLOR'].nil?
       end
 
       # Overwrite show_diff to show diff with colors if Diff::LCS is

spec/shell/color_spec.rb

@@ -19,6 +19,15 @@ def shell
       shell.ask "Is this green?", :green, :limited_to => %w(Yes No Maybe)
     end
 
+    it "does not set the color if specified and NO_COLOR is set" do
+      allow(ENV).to receive(:[]).with("NO_COLOR").and_return("")
+      expect(Thor::LineEditor).to receive(:readline).with("Is this green? ", anything).and_return("yes")
+      shell.ask "Is this green?", :green
+
+      expect(Thor::LineEditor).to receive(:readline).with("Is this green? [Yes, No, Maybe] ", anything).and_return("Yes")
+      shell.ask "Is this green?", :green, :limited_to => %w(Yes No Maybe)
+    end
+
     it "handles an Array of colors" do
       expect(Thor::LineEditor).to receive(:readline).with("\e[32m\e[47m\e[1mIs this green on white? \e[0m", anything).and_return("yes")
       shell.ask "Is this green on white?", [:green, :on_white, :bold]
@@ -48,6 +57,15 @@ def shell
       expect(out.chomp).to eq("Wow! Now we have colors!")
     end
 
+    it "does not set the color if NO_COLOR is set" do
+      allow(ENV).to receive(:[]).with("NO_COLOR").and_return("")
+      out = capture(:stdout) do
+        shell.say "Wow! Now we have colors!", :green
+      end
+
+      expect(out.chomp).to eq("Wow! Now we have colors!")
+    end
+
     it "does not use a new line even with colors" do
       out = capture(:stdout) do
         shell.say "Wow! Now we have colors! ", :green
@@ -118,6 +136,13 @@ def shell
       colorless = shell.set_color "hi!", :white
       expect(colorless).to eq("hi!")
     end
+
+    it "does nothing when the terminal has the NO_COLOR environment variable set" do
+      allow(ENV).to receive(:[]).with("NO_COLOR").and_return("")
+      allow($stdout).to receive(:tty?).and_return(true)
+      colorless = shell.set_color "hi!", :white
+      expect(colorless).to eq("hi!")
+    end
   end
 
   describe "#file_collision" do