Allow icons to be omitted by setting false

[frankieroberto]

If you don't want to use the default GOVUK icons but don't want to create your own (particularly the .ico format one), you should be able to turn them off by setting them to false:

icons: {
  mask: false,
  shortcut: false,
  touch: false        
}

Not sure this needs to be advertised in the docs, as the plugin is mainly designed to be used for sites that do carry the govuk branding.