config: ArchPackageManager should be snake_case (#784)

* config: ArchPackageManager should be snake_case

* Remove unnecessary strum macro

* Add arch_package_manager to config.example.toml

src/config.rs

@@ -181,6 +181,7 @@ pub struct Brew {
 }
 
 #[derive(Debug, Deserialize, Clone, Copy)]
+#[serde(rename_all = "snake_case")]
 pub enum ArchPackageManager {
     Autodetect,
     Trizen,