Add rule widget

Adds a new rule widget.
Also fixes some bugs I encountered while testing
some unrelated things in an extremely small console.

src/Spectre.Console/Extensions/RuleExtensions.cs

@@ -0,0 +1,75 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="RuleExtensions"/>.
+    /// </summary>
+    public static class RuleExtensions
+    {
+        /// <summary>
+        /// Sets the rule title.
+        /// </summary>
+        /// <param name="rule">The rule.</param>
+        /// <param name="title">The title.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static Rule SetTitle(this Rule rule, string title)
+        {
+            if (rule is null)
+            {
+                throw new ArgumentNullException(nameof(rule));
+            }
+
+            if (title is null)
+            {
+                throw new ArgumentNullException(nameof(title));
+            }
+
+            rule.Title = title;
+            return rule;
+        }
+
+        /// <summary>
+        /// Sets the rule style.
+        /// </summary>
+        /// <param name="rule">The rule.</param>
+        /// <param name="style">The rule style string.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static Rule SetStyle(this Rule rule, string style)
+        {
+            if (rule is null)
+            {
+                throw new ArgumentNullException(nameof(rule));
+            }
+
+            if (style is null)
+            {
+                throw new ArgumentNullException(nameof(style));
+            }
+
+            return SetStyle(rule, Style.Parse(style));
+        }
+
+        /// <summary>
+        /// Sets the rule style.
+        /// </summary>
+        /// <param name="rule">The rule.</param>
+        /// <param name="style">The rule style.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static Rule SetStyle(this Rule rule, Style style)
+        {
+            if (rule is null)
+            {
+                throw new ArgumentNullException(nameof(rule));
+            }
+
+            if (style is null)
+            {
+                throw new ArgumentNullException(nameof(style));
+            }
+
+            rule.Style = style;
+            return rule;
+        }
+    }
+}