Add "storm" option for the span property

[webbati]

Now is not possible to set "storm" as span value and the builder plugin delete the value saving after you set it manually.

We also apply the "cssClass" property to the widget of the builder if "storm" value is selected for the span property, default class "col-sx-12 col-md-6" will be applied if nothing is set

[LukeTowers]

We should probably call it manual instead, storm isn't really a special value that's treated any differently if I recall, it's just the use of any non-supported values that triggers the behaviours associated with setting span: storm.

[bennothommo]

@LukeTowers you're right, also, it looks like auto doesn't do anything either - at the very least, I haven't found any code that directly targets it.