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/Cli/CommandSettings.cs

@@ -0,0 +1,17 @@
+namespace Spectre.Console.Cli
+{
+    /// <summary>
+    /// Base class for command settings.
+    /// </summary>
+    public abstract class CommandSettings
+    {
+        /// <summary>
+        /// Performs validation of the settings.
+        /// </summary>
+        /// <returns>The validation result.</returns>
+        public virtual ValidationResult Validate()
+        {
+            return ValidationResult.Success();
+        }
+    }
+}