feat: Add -casesensitive flag

[gabe565]

This is one of my two solutions to fix #40.

Add a -casesensitive flag, which disables all generation of lower-cased values.

• This required more code changes up front, and users have to know to add the -casesensitive flag, but the resulting functionality completely eliminates the possibility of choosing the incorrect value.
• This method also has the benefit of not changing existing code after users update enumer. Code would only change after adding -casesensitive.
• Unfortunately, users may not realize there is a -casesensitive flag and could introduce bugs in their code in the future.

Fixes #40