Optional env

[pleshevskiy]

Add possibility to get optional env

itconfig::config! {
     SOMETHING: Option<&'static str>,
     NOTHING: Option<&'static str>,
}

fn main() {
    std::env::set_var("SOMETHING", "hello world");
    assert_eq!(config::SOMETHING(), Some("hello world"));
    assert_eq!(config::NOTHING(), None);
}

small internal changes:

• Move helper functions to utils
• Deny all clippy rules