Add support for lowerCamelCase enum cases

[bmunge]

The accepted Swift evolution guideline for Swift 3.0 standardizes lowerCamelCase enum values.
Source

Unless I'm missing something, the current type_name rule does not allow a way to ignore the uppercase requirement for enum cases. I think support needs to be added to the type_name rule to exclude enum cases from the capitalization requirement, or from the rule entirely.

[jpsim]

as of #660, we allow lowercase names for enum cases.