Give more control on the serialization of macro parameter of type enum

Currently, enum/edit.vm is used to generate the form field of macro parameters of type Enum.

The .name() method is used to generate the values of the different enum options.

Those values are usually uppercase (but the EnumConverter is case insensitive), so while it's possible to write them in lower case in wiki syntax, they are uppercase using CKEditor for instance.

It could be nicer to provide a way to allow these options to be lower-cased by default.