Add Spectre.Cli to Spectre.Console

* Renames Spectre.Cli to Spectre.Console.Cli.
* Now uses Verify with Spectre.Console.Cli tests.
* Removes some duplicate definitions.

Closes #168

src/Spectre.Console.Tests/Data/Validators/EvenNumberValidatorAttribute.cs

@@ -0,0 +1,29 @@
+using System;
+using Spectre.Console.Cli;
+
+namespace Spectre.Console.Tests.Data
+{
+    [AttributeUsage(AttributeTargets.Property, AllowMultiple = true)]
+    public sealed class EvenNumberValidatorAttribute : ParameterValidationAttribute
+    {
+        public EvenNumberValidatorAttribute(string errorMessage)
+            : base(errorMessage)
+        {
+        }
+
+        public override ValidationResult Validate(ICommandParameterInfo info, object value)
+        {
+            if (value is int integer)
+            {
+                if (integer % 2 == 0)
+                {
+                    return ValidationResult.Success();
+                }
+
+                return ValidationResult.Error($"Number is not even ({info?.PropertyName}).");
+            }
+
+            throw new InvalidOperationException($"Parameter is not a number ({info?.PropertyName}).");
+        }
+    }
+}