Add option to `AtatEnum` when deriving to automatically derive default

[MathiasKoch]

When deriving AtatEnum for an enum, i think it would make sense to be able to mark the default value (factory-programmed value) as:

/// GPIO input value (for input function <gpio_mode>=1 only):
#[derive(Clone, PartialEq, AtatEnum)]
pub enum GpioInPull {
    /// (default value): no resistor activated
    #[at_arg(default, value = 0, position = 0)
    NoPull,
    /// pull up resistor active
    #[at_arg(value = 1, position = 1)
    PullUp,
    /// pull down resistor active
    #[at_arg(value = 2, position = 2)
    PullDown,
}

Expanding the default part of #[at_arg(default, value = 0, position = 0) to:

impl Default for GpioInPull {
    fn default() -> Self {
        GpioInPull::NoPull
    }
}