Added option to use custom icon classes

[maximilianmeier]

We needed the option to add a custom icon based on our icon font.

So we added the option to add custom icon classes over the environment.js

'ember-cli-notifications': {
  icons: 'custom',
  successIcon: 'successIcon-Class',
  infoIcon: 'infoIcon-Class',
  warningIcon: 'warningIcon-Class',
  errorIcon: 'errorIcon-Class',
  closeIcon: 'closeIcon-Class',
},