make some target _blank configurable

there seem to be "only" 2 places where this make sens, the other
being href in mdcell, detach pager, help link in new windows.

set IPython._target = '_self' for try.jupyter.org shoudl work in
custom.js

IPython/html/static/tree/js/notebooklist.js

@@ -293,7 +293,7 b' define(['
         // directory nav doesn't open new tabs
         // files, notebooks do
         if (model.type !== "directory") {
-            link.attr('target','_blank');
+            link.attr('target',IPython._target||'_blank');
         }
         if (model.type !== 'directory') {
             this.add_duplicate_button(item);

IPython/html/static/tree/js/terminallist.js

@@ -93,7 +93,7 b' define(['
         item.find(".item_icon").addClass("fa fa-terminal");
         var link = item.find("a.item_link")
             .attr('href', utils.url_join_encode(this.base_url, "terminals", name));
-        link.attr('target', '_blank');
+        link.attr('target', IPython._target||'_blank');
         this.add_shutdown_button(name, item);
     };