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/Annotations/CommandArgumentAttribute.cs

@@ -0,0 +1,54 @@
+using System;
+using Spectre.Console.Cli.Internal;
+
+namespace Spectre.Console.Cli
+{
+    /// <summary>
+    /// An attribute representing a command argument.
+    /// </summary>
+    /// <seealso cref="Attribute" />
+    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
+    public sealed class CommandArgumentAttribute : Attribute
+    {
+        /// <summary>
+        /// Gets the argument position.
+        /// </summary>
+        /// <value>The argument position.</value>
+        public int Position { get; }
+
+        /// <summary>
+        /// Gets the value name of the argument.
+        /// </summary>
+        /// <value>The value name of the argument.</value>
+        public string ValueName { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether the argument is required.
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> if the argument is required; otherwise, <c>false</c>.
+        /// </value>
+        public bool IsRequired { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CommandArgumentAttribute"/> class.
+        /// </summary>
+        /// <param name="position">The argument position.</param>
+        /// <param name="template">The argument template.</param>
+        public CommandArgumentAttribute(int position, string template)
+        {
+            if (template == null)
+            {
+                throw new ArgumentNullException(nameof(template));
+            }
+
+            // Parse the option template.
+            var result = TemplateParser.ParseArgumentTemplate(template);
+
+            // Assign the result.
+            Position = position;
+            ValueName = result.Value;
+            IsRequired = result.Required;
+        }
+    }
+}