Automatic table of contents generation for Markdown

Adds table of contents for any Markdown note or
Markdown snippet.
Consequent generations update existing TOC.
Generated TOC is case sensitive to handle #2067

Shortcut : CommandOrControl+Alt+T
Menu : Edit/Generate/Update Markdown TOC

lib/main-menu.js

@@ -228,6 +228,16 @@ const edit = {
       click () {
         mainWindow.webContents.send('editor:add-tag')
       }
+    },
+    {
+      type: 'separator'
+    },
+    {
+      label: 'Generate/Update Markdown TOC',
+      accelerator: 'CommandOrControl+Alt+T',
+      click () {
+        mainWindow.webContents.send('code:generate-toc')
+      }
     }
   ]
 }