Function envoption::get

pub fn get<T, B>(
    var_name: &str, 
    mode: OptionType<B>
) -> Result<Option<T>, EnvOptionError<T::Err>> where
    B: Into<T>,
    T: FromStr + Debug,
    T::Err: Error, 

Get an environment variable, using the given mode to determine behavior when it is not set.