Add support for adding empty rows

This affects grids and tables.

src/Spectre.Console/Composition/Grid.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Spectre.Console.Composition;
+using Spectre.Console.Internal;
 
 namespace Spectre.Console
 {
@@ -56,6 +58,11 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(column));
             }
 
+            if (_table.RowCount > 0)
+            {
+                throw new InvalidOperationException("Cannot add new columns to grid with existing rows.");
+            }
+
             // Only pad the most right cell if we've explicitly set a padding.
             _table.PadRightCell = column.Padding != null;
 
@@ -97,6 +104,16 @@ namespace Spectre.Console
             }
         }
 
+        /// <summary>
+        /// Adds an empty row to the grid.
+        /// </summary>
+        public void AddEmptyRow()
+        {
+            var columns = new string[_table.ColumnCount];
+            Enumerable.Range(0, _table.ColumnCount).ForEach(index => columns[index] = string.Empty);
+            AddRow(columns);
+        }
+
         /// <summary>
         /// Adds a new row to the grid.
         /// </summary>

src/Spectre.Console/Composition/Table.cs

@@ -81,7 +81,7 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(column));
             }
 
-            _columns.Add(new TableColumn(column));
+            AddColumn(new TableColumn(column));
         }
 
         /// <summary>
@@ -95,6 +95,11 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(column));
             }
 
+            if (_rows.Count > 0)
+            {
+                throw new InvalidOperationException("Cannot add new columns to table with existing rows.");
+            }
+
             _columns.Add(column);
         }
 
@@ -109,7 +114,10 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(columns));
             }
 
-            _columns.AddRange(columns.Select(column => new TableColumn(column)));
+            foreach (var column in columns)
+            {
+                AddColumn(column);
+            }
         }
 
         /// <summary>
@@ -123,7 +131,20 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(columns));
             }
 
-            _columns.AddRange(columns.Select(column => column));
+            foreach (var column in columns)
+            {
+                AddColumn(column);
+            }
+        }
+
+        /// <summary>
+        /// Adds an empty row to the table.
+        /// </summary>
+        public void AddEmptyRow()
+        {
+            var columns = new string[ColumnCount];
+            Enumerable.Range(0, ColumnCount).ForEach(index => columns[index] = string.Empty);
+            AddRow(columns);
         }
 
         /// <summary>