refactor: Create executor struct to get rid of global variables

Maybe eventually help on #17

errors.go

@@ -1,9 +1,17 @@
 package task
 
 import (
+	"errors"
 	"fmt"
 )
 
+var (
+	// ErrCyclicDependencyDetected is returned when a cyclic dependency was found in the Taskfile
+	ErrCyclicDependencyDetected = errors.New("task: cyclic dependency detected")
+	// ErrTaskfileAlreadyExists is returned on creating a Taskfile if one already exists
+	ErrTaskfileAlreadyExists = errors.New("task: A Taskfile already exists")
+)
+
 type taskFileNotFound struct {
 	taskFile string
 }