Turndown: remove \n in table cell

When pasting table from OneNote, there may be redundant \n that will
break the table.

src/utils/turndown/turndown-plugin-gfm.js

@@ -44,6 +44,7 @@ var configs = { autoHead: false };
 rules.tableCell = {
   filter: ['th', 'td'],
   replacement: function (content, node) {
+    content = content.replace(/[\r\n]/g, '');
     return cell(content, node)
   }
 };