Implement global.rsync_options (fixes #206)

Signed-off-by: Harry Chen <i@harrychen.xyz>

worker/config.go

@@ -58,6 +58,9 @@ type globalConfig struct {
 	Retry      int    `toml:"retry"`
 	Timeout    int    `toml:"timeout"`
 
+	// appended to the options generated by rsync_provider, but before mirror-specific options
+	RsyncOptions []string `toml:"rsync_options"`
+
 	ExecOnSuccess []string `toml:"exec_on_success"`
 	ExecOnFailure []string `toml:"exec_on_failure"`
 }
@@ -162,7 +165,7 @@ type mirrorConfig struct {
 	ExecOnSuccess []string `toml:"exec_on_success"`
 	ExecOnFailure []string `toml:"exec_on_failure"`
 
-	// These two options  the global options
+	// These two options are appended to the global options
 	ExecOnSuccessExtra []string `toml:"exec_on_success_extra"`
 	ExecOnFailureExtra []string `toml:"exec_on_failure_extra"`