Feature/vim flavoured jsbin

I added vim flavoured option at vim.jsbin.com. All the editors will have vim keymap after loading. 

To achieve this besides adding `vim` folder inside `custom` it was needed to include a custom js script into the index.html template so the necessary modifications to `lib/handlers/bin.js` and `views/index.html`.

In order to avoid including multiple js files directly into the index view the decision was to load dynamically the `keymap/vim.js` file and then apply the vim keymap to all editors.