Add option to set the primary output

This closes #4

Author: fd0

doc/grobi.conf

@@ -11,6 +11,7 @@ rules:
         - HDMI2
         - HDMI3
     atomic: true
+    primary: HDMI3
 
   - name: Mobile
     outputs_disconnected:

src/cmd/grobi/randr.go

@@ -370,6 +370,10 @@ func BuildCommandOutputRow(rule Rule, current Outputs) ([]*exec.Cmd, error) {
 			args = append(args, "--right-of", lastOutput)
 		}
 
+		if rule.Primary == name {
+			args = append(args, "--primary")
+		}
+
 		lastOutput = name
 		enableOutputArgs = append(enableOutputArgs, args)
 	}

src/cmd/grobi/rule.go

@@ -13,6 +13,8 @@ type Rule struct {
 	ConfigureSingle  string   `yaml:"configure_single"`
 	ConfigureCommand string   `yaml:"configure_command"`
 
+	Primary string `yaml:"primary"`
+
 	DisableOrder []string `yaml:"disable_order"`
 
 	Atomic bool `yaml:"atomic"`