Allow case insensitivity in arg_enum! macro

[kbknapp]

Currently if you use arg_enum! to create possible values into an enum, you must either allow non-camel-case enum variants (to suppress the warning), or force users to start their possible values with an upper case character. For example, if you had a --mode which accepted an arg_enum! of

arg_enum! {
    enum {
        Vi,
        Emacs
    }
}

The user must use $ myprog --mode Vi which isn't optimal. This enum should be able to match vi or Vi.

Simply converting to lowercase is also a very tricky subject if utf-8 is to be supported. But being, I don't use any non-ascii characters on a regular basis, I'm unfamiliar with capital/lowercase unicode graphemes.