Add support for rows

Closes #69

src/Spectre.Console/Widgets/Rows.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Renders things in rows.
+    /// </summary>
+    public sealed class Rows : Renderable, IExpandable
+    {
+        private readonly List<IRenderable> _children;
+
+        /// <inheritdoc/>
+        public bool Expand { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Rows"/> class.
+        /// </summary>
+        /// <param name="children">The children to render.</param>
+        public Rows(IEnumerable<IRenderable> children)
+        {
+            _children = new List<IRenderable>(children ?? throw new ArgumentNullException(nameof(children)));
+        }
+
+        /// <inheritdoc/>
+        protected override Measurement Measure(RenderContext context, int maxWidth)
+        {
+            if (Expand)
+            {
+                return new Measurement(maxWidth, maxWidth);
+            }
+            else
+            {
+                var measurements = _children.Select(c => c.Measure(context, maxWidth));
+                return new Measurement(
+                    measurements.Min(c => c.Min),
+                    measurements.Min(c => c.Max));
+            }
+        }
+
+        /// <inheritdoc/>
+        protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
+        {
+            foreach (var child in _children)
+            {
+                var segments = child.Render(context, maxWidth);
+                foreach (var (_, _, last, segment) in segments.Enumerate())
+                {
+                    yield return segment;
+
+                    if (last)
+                    {
+                        if (!segment.IsLineBreak)
+                        {
+                            yield return Segment.LineBreak;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}