Restructure solution a bit

src/Spectre.Console/IExpandable.cs

@@ -0,0 +1,15 @@
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents something that is expandable.
+    /// </summary>
+    public interface IExpandable
+    {
+        /// <summary>
+        /// Gets or sets a value indicating whether or not the object should
+        /// expand to the available space. If <c>false</c>, the object's
+        /// width will be auto calculated.
+        /// </summary>
+        bool Expand { get; set; }
+    }
+}