Allow color numbers in markup expressions

Closes #614

src/Spectre.Console/StyleParser.cs

@@ -96,6 +96,21 @@ namespace Spectre.Console
                                 return null;
                             }
                         }
+                        else if (int.TryParse(part, out var number))
+                        {
+                            if (number < 0)
+                            {
+                                error = $"Color number must be greater than or equal to 0 (was {number})";
+                                return null;
+                            }
+                            else if (number > 255)
+                            {
+                                error = $"Color number must be less than or equal to 255 (was {number})";
+                                return null;
+                            }
+
+                            color = number;
+                        }
                         else
                         {
                             error = !foreground

test/Spectre.Console.Tests/Unit/StyleTests.cs

@@ -275,6 +275,31 @@ namespace Spectre.Console.Tests.Unit
                 result.Background.ShouldBe(Color.Blue);
             }
 
+            [Theory]
+            [InlineData("12 on 24")]
+            public void Should_Parse_Colors_Numbers_Correctly(string style)
+            {
+                // Given, When
+                var result = Style.Parse(style);
+
+                // Then
+                result.Foreground.ShouldBe(Color.Blue);
+                result.Background.ShouldBe(Color.DeepSkyBlue4_1);
+            }
+
+            [Theory]
+            [InlineData("-12", "Color number must be greater than or equal to 0 (was -12)")]
+            [InlineData("256", "Color number must be less than or equal to 255 (was 256)")]
+            public void Should_Return_Error_If_Color_Number_Is_Invalid(string style, string expected)
+            {
+                // Given, When
+                var result = Record.Exception(() => Style.Parse(style));
+
+                // Then
+                result.ShouldNotBeNull();
+                result.Message.ShouldBe(expected);
+            }
+
             [Theory]
             [InlineData("rgb()", "Invalid RGB color 'rgb()'.")]
             [InlineData("rgb(", "Invalid RGB color 'rgb('.")]