Support down-converting of HTML to plain text

- Display a warning when text was generated from HTML
- Add a semi complicated, responsive HTML email for future testing
- Closes #20

themes/bootstrap/templates/mailbox/_show.html

@@ -60,6 +60,13 @@
   </div>
 </div>
 
+{{if .isTextFromHTML}}
+<div class="alert alert-warning" role="alert">
+  <strong>Notice:</strong> Message did not contain a plain text portion;
+  the text below was converted from HTML automatically.
+</div>
+{{end}}
+
 <div class="message-body">{{.body}}</div>
 
 {{with .attachments}}