Custom icons

I want to add a custom icon to the grapesjs panel at the top of the editor page. Is it possible to do this with any SVG or with text? If so, can you explain how? I've tried the following but couldn't get an icon to show:

var pnm = editor.Panels;
pnm.addButton('options', {
  label: '<svg ... </svg>',
 ...
});

I've also tried this to use text as an icon, but couldn't get that working either:

var pnm = editor.Panels;
pnm.addButton('options', {
  label: 'words',
  className: 'fa fa-text',
 ...
});

(I'm also using the newsletter plugin).