fix(CIcon): fix class order and incorrect class name, update docs

[klaseca]

This pull request include:

Fixed incorrect class order

Now if we pass a size property with a value of custom-size, we do not get expected class (icon-custom-size), because it is further excluded by other conditions (lack of width or height)

Fixed incorrect class name

Class icon-c-s is currently used, although it should be used icon-custom (according to declared types)

Updated docs

Added instructions for using css