Add configuration flags for portage arguments (#436)

config.example.toml

@@ -58,6 +58,8 @@
 #yay_arguments = "--nodevel"
 #trizen_arguments = "--devel"
 #enable_tlmgr = true
+emerge_sync_flags = "-q"
+emerge_update_flags = "-uDNa --with-bdeps=y world"
 
 #[windows]
 # Manually select Windows updates