Add support for remapping emojis

Closes #94

docs/input/appendix/emojis.md

@@ -30,6 +30,25 @@ var phrase = "Mmmm :birthday_cake:";
 var rendered = Emoji.Replace(phrase);
 ```
 
+# Remapping or adding an emoji
+
+Sometimes you want to remap an existing emoji, or 
+add a completely new one. For this you can use the 
+`Emoji.Remap` method. This approach works both with 
+markup strings and `Emoji.Replace`.
+
+```csharp
+// Remap the emoji
+Emoji.Remap("globe_showing_europe_africa", "😄");
+
+// Render markup
+AnsiConsole.MarkupLine("Hello :globe_showing_europe_africa:!");
+
+// Replace emojis in string
+var phrase = "Hello :globe_showing_europe_africa:!";
+var rendered = Emoji.Replace(phrase);
+```
+
 # Emojis
 
 _The images in the table below might not render correctly in your 

examples/Emojis/Program.cs

@@ -6,11 +6,19 @@ namespace EmojiExample
     {
         public static void Main(string[] args)
         {
-            // Markup
+            // Show a known emoji
+            RenderEmoji();
+
+            // Show a remapped emoji
+            Emoji.Remap("globe_showing_europe_africa", Emoji.Known.GrinningFaceWithSmilingEyes);
+            RenderEmoji();
+        }
+
+        private static void RenderEmoji()
+        {
             AnsiConsole.Render(
                 new Panel("[yellow]Hello :globe_showing_europe_africa:![/]")
-                    .RoundedBorder()
+                    .RoundedBorder());
-                    .SetHeader("Markup"));
         }
     }
 }

src/.editorconfig

@@ -87,3 +87,6 @@ dotnet_diagnostic.RCS1227.severity = none
 
 # IDE0004: Remove Unnecessary Cast
 dotnet_diagnostic.IDE0004.severity = warning
+
+# CA1810: Initialize reference type static fields inline
+dotnet_diagnostic.CA1810.severity = none

src/Spectre.Console/Emoji.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using System.Text.RegularExpressions;
 
 namespace Spectre.Console
@@ -8,6 +10,35 @@ namespace Spectre.Console
     public static partial class Emoji
     {
         private static readonly Regex _emojiCode = new Regex(@"(:(\S*?):)", RegexOptions.Compiled);
+        private static readonly Dictionary<string, string> _remappings;
+
+        static Emoji()
+        {
+            _remappings = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        }
+
+        /// <summary>
+        /// Remaps a specific emoji tag with a new emoji.
+        /// </summary>
+        /// <param name="tag">The emoji tag.</param>
+        /// <param name="emoji">The emoji.</param>
+        public static void Remap(string tag, string emoji)
+        {
+            if (tag is null)
+            {
+                throw new ArgumentNullException(nameof(tag));
+            }
+
+            if (emoji is null)
+            {
+                throw new ArgumentNullException(nameof(emoji));
+            }
+
+            tag = tag.TrimStart(':').TrimEnd(':');
+            emoji = emoji.TrimStart(':').TrimEnd(':');
+
+            _remappings[tag] = emoji;
+        }
 
         /// <summary>
         /// Replaces emoji markup with corresponding unicode characters.
@@ -19,6 +50,12 @@ namespace Spectre.Console
             static string ReplaceEmoji(Match match)
             {
                 var key = match.Groups[2].Value;
+
+                if (_remappings.Count > 0 && _remappings.TryGetValue(key, out var remappedEmoji))
+                {
+                    return remappedEmoji;
+                }
+
                 if (_emojis.TryGetValue(key, out var emoji))
                 {
                     return emoji;