Allow option to limit icon production

[JasonGoemaat]

I would like to produce only the icon types and sizes I want as part of a build process, but I can't figure out how to limit that with the options object. The favicon object seems to let you create all the png sizes you want, but it always creates 'favicon.ico' which I don't want...

  favicon: {
    name: 'icons-',
    pngSizes: [16, 48, 128],
    icoSizes: []
  }

[isaiahtaylor]

I'm also not seeing away to override the name or otherwise prevent the generation of the favicon.ico file

[panther7]

#155