Change the spell checker support by:

1. Including the spell checker definition assembly in the vsix
1. Creating a custom natural text tag for markdown files
1. No longer having the "markdown" content type inherit from "plaintext"

ContentType.cs

@@ -10,7 +10,6 @@ static class ContentType
         [Export]
         [Name(Name)]
         [DisplayName("Markdown")]
-        [BaseDefinition("plaintext")]
         [BaseDefinition("HTML")]
         public static ContentTypeDefinition MarkdownModeContentType = null;
 

MarkdownMode.csproj

@@ -88,6 +88,7 @@
       <AutoGen>True</AutoGen>
       <DesignTime>True</DesignTime>
     </Compile>
+    <Compile Include="SpellcheckSupport\MarkdownNaturalTextTagger.cs" />
     <Compile Include="ToolWindow\Guids.cs" />
     <Compile Include="ToolWindow\PreviewWindowUpdateListener.cs" />
     <Compile Include="ToolWindow\Package.cs" />
@@ -128,6 +129,12 @@
       <MergeWithCTO>true</MergeWithCTO>
     </EmbeddedResource>
   </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="Spellchecker\SpellChecker.Definitions\SpellChecker.Definitions.csproj">
+      <Project>{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}</Project>
+      <Name>SpellChecker.Definitions</Name>
+    </ProjectReference>
+  </ItemGroup>
   <PropertyGroup>
     <RegisterOutputPackage>true</RegisterOutputPackage>
     <RegisterWithCodebase>true</RegisterWithCodebase>

MarkdownMode.sln

@@ -3,6 +3,8 @@ Microsoft Visual Studio Solution File, Format Version 11.00
 # Visual Studio 2010
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MarkdownMode", "MarkdownMode.csproj", "{1017287A-83B1-41AB-BAC2-045AC45607C7}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SpellChecker.Definitions", "Spellchecker\SpellChecker.Definitions\SpellChecker.Definitions.csproj", "{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -13,6 +15,10 @@ Global
 		{1017287A-83B1-41AB-BAC2-045AC45607C7}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{1017287A-83B1-41AB-BAC2-045AC45607C7}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{1017287A-83B1-41AB-BAC2-045AC45607C7}.Release|Any CPU.Build.0 = Release|Any CPU
+		{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{87D22AC6-424B-48DD-A5E8-DCB7CB3DDD63}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

SpellcheckSupport/MarkdownNaturalTextTagger.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using SpellChecker.Definitions;
+using Microsoft.VisualStudio.Text;
+using Microsoft.VisualStudio.Text.Tagging;
+using System.ComponentModel.Composition;
+using Microsoft.VisualStudio.Utilities;
+
+namespace MarkdownMode.SpellcheckSupport
+{
+    [Export(typeof(ITaggerProvider))]
+    [ContentType(ContentType.Name)]
+    [TagType(typeof(NaturalTextTag))]
+    class MarkdownNaturalTextTaggerProvider : ITaggerProvider
+    {
+        public ITagger<T> CreateTagger<T>(ITextBuffer buffer) where T : ITag
+        {
+            return new MarkdownNaturalTextTagger() as ITagger<T>;
+        }
+    }
+
+    class MarkdownNaturalTextTagger : ITagger<NaturalTextTag>
+    {
+        public IEnumerable<ITagSpan<NaturalTextTag>> GetTags(NormalizedSnapshotSpanCollection spans)
+        {
+            foreach (var snapshotSpan in spans)
+            {
+                yield return new TagSpan<NaturalTextTag>(snapshotSpan, new NaturalTextTag());
+            }
+        }
+
+#pragma warning disable 67
+        public event EventHandler<SnapshotSpanEventArgs> TagsChanged;
+#pragma warning restore 67
+    }
+
+    class NaturalTextTag : INaturalTextTag { }
+}