Add the ability to create icon aliases

The current framework uses the name of the icon to represent class names in the generated stylesheet. In many cases, the icon name is based on a description of the icon. Often, users are more interested in how the icon will be used which is more descriptive in the markup. For example, the cog icon is often used in the UI to represent settings. It should be possible to have a settings class which is an alias for the cog class.