add hooks to execute scripts when ready and on modifications

Replace cmd key in the config with hooks.modified and define one more optional commands to execute when ready in case we don't use s6 readiness or we use rconfd in one shot mode.

#[derive(Debug, Deserialize)]
pub struct Hooks {
    /// executed whenever some files have been modified
    pub modified: Option<String>,
    /// executed right after the first manifestation
    pub ready: Option<String>
}