Add cache_ignore variant for cacheable enum

[AndreyErmilov]

Linked to #35
For case you need to cache some variants of enum we need macro parameter #[cache_ignore].

Example:

pub enum MyEnum {
    Success1(i32),
    Success2(i32),
    #[cache_ignore]
    Fail1(i32),
    #[cache_ignore]
    Fail2(i32),
}