[toc2] add a tag #skip to skip headers from being inserted in toc, cf #1027

Author: jfbercher

src/jupyter_contrib_nbextensions/nbextensions/toc2/toc2.js

@@ -550,6 +550,10 @@ var table_of_contents = function (cfg,st) {
       if (h.id=="Table-of-Contents"){ return; }
       //If h had already a number, remove it
       $(h).find(".toc-item-num").remove();
+      // skip header if tag #skip is present
+      if (h.id.search('#skip') !== -1 ) {
+          $(h).text($(h).text().replace("#skip", ""));
+          return; }
       var num_str= incr_lbl(lbl_ary,level-1).join('.');// numbered heading labels
       var num_lbl= $("<span/>").addClass("toc-item-num")
             .text(num_str).append('&nbsp;').append('&nbsp;');