Implement GPA score

I'm trying to follow the [GPA (grade point average)](https://bigfuture.collegeboard.org/plan-for-college/college-basics/how-to-convert-gpa-4.0-scale)
but the example there used the 4.0 scale, and I'm not pretty sure if we
want that.

Closes #95

Author: JuanVqz

lib/skunk/cli/grade_point_average.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module Skunk
+  module Cli
+    class GradePointAverage
+      def initialize(skunk_score)
+        @skunk_score = skunk_score
+      end
+
+      def score
+        case @skunk_score
+        when (0..64)
+          "A+"
+        when (65..66)
+          "A"
+        when (67..69)
+          "A-"
+        when (70..72)
+          "B+"
+        when (73..76)
+          "B"
+        when (77..79)
+          "B-"
+        when (80..82)
+          "C+"
+        when (83..86)
+          "C"
+        when (87..89)
+          "C-"
+        when (90..92)
+          "D+"
+        when (93..96)
+          "D"
+        when (93..Float::INFINITY)
+          "E"
+        end
+      end
+    end
+  end
+end

test/lib/skunk/cli/grade_point_average_test.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+=begin
+|Letter Grade | Percent Grade | Scale |
+|:------------|:-------------:|------:|
+| A+          | Below 65      | 4.0   |
+| A           | 65-66         | 4.0   |
+| A-          | 67-69         | 3.7   |
+| B+          | 70-72         | 3.3   |
+| B           | 73-76         | 3.0   |
+| B-          | 77-79         | 2.7   |
+| C+          | 80-82         | 2.3   |
+| C           | 83-86         | 2.0   |
+| C-          | 87-89         | 1.7   |
+| D+          | 90-92         | 1.3   |
+| D           | 93-96         | 1.0   |
+| E/F         | 97-100        | 0.0   |
+=end
+
+require "test_helper"
+
+require "skunk/cli/grade_point_average"
+
+describe Skunk::Cli::GradePointAverage do
+  subject { Skunk::Cli::GradePointAverage }
+
+  describe "#score" do
+    context "with A score" do
+      it { expect(subject.new(0).score).must_equal "A+" }
+      it { expect(subject.new(65).score).must_equal "A" }
+      it { expect(subject.new(66).score).must_equal "A" }
+      it { expect(subject.new(67).score).must_equal "A-" }
+      it { expect(subject.new(69).score).must_equal "A-" }
+    end
+
+    context "with B score" do
+      it { expect(subject.new(70).score).must_equal "B+" }
+      it { expect(subject.new(72).score).must_equal "B+" }
+      it { expect(subject.new(73).score).must_equal "B" }
+      it { expect(subject.new(76).score).must_equal "B" }
+      it { expect(subject.new(77).score).must_equal "B-" }
+      it { expect(subject.new(79).score).must_equal "B-" }
+    end
+
+    context "with C score" do
+      it { expect(subject.new(80).score).must_equal "C+" }
+      it { expect(subject.new(82).score).must_equal "C+" }
+      it { expect(subject.new(83).score).must_equal "C" }
+      it { expect(subject.new(86).score).must_equal "C" }
+      it { expect(subject.new(87).score).must_equal "C-" }
+      it { expect(subject.new(89).score).must_equal "C-" }
+    end
+
+    context "with D score" do
+      it { expect(subject.new(90).score).must_equal "D+" }
+      it { expect(subject.new(92).score).must_equal "D+" }
+      it { expect(subject.new(93).score).must_equal "D" }
+      it { expect(subject.new(96).score).must_equal "D" }
+    end
+
+    context "with E score" do
+      it { expect(subject.new(97).score).must_equal "E" }
+      it { expect(subject.new(1000).score).must_equal "E" }
+      it { expect(subject.new(10000).score).must_equal "E" }
+    end
+  end
+end