Allow global variables in the Taskfile

Closes #66

internal/taskfile/taskfile.go

@@ -3,6 +3,7 @@ package taskfile
 // Taskfile represents a Taskfile.yml
 type Taskfile struct {
 	Version string
+	Vars    Vars
 	Tasks   Tasks
 }
 
@@ -15,12 +16,14 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 
 	var taskfile struct {
 		Version string
+		Vars    Vars
 		Tasks   Tasks
 	}
 	if err := unmarshal(&taskfile); err != nil {
 		return err
 	}
 	tf.Version = taskfile.Version
+	tf.Vars = taskfile.Vars
 	tf.Tasks = taskfile.Tasks
 	return nil
 }