Remove example
Add NERDTree statusline

This *almost* works. There are a couple of issues that needs to be
worked out:

- It's currently not possible to inject the theme into the
  ext.vim.local_themes dict, and the fix in this statusline overrides
  existing config overrides
- It's not possible to add the plugin search path to Powerline after
  Powerline has been loaded, so the nerdtree theme file must be copied
  to an existing search path in order to work