Add hability to globally set environment variables

Closes #138

internal/taskfile/taskfile.go

@@ -7,6 +7,7 @@ type Taskfile struct {
 	Output     string
 	Includes   map[string]string
 	Vars       Vars
+	Env        Vars
 	Tasks      Tasks
 }
 
@@ -23,6 +24,7 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		Output     string
 		Includes   map[string]string
 		Vars       Vars
+		Env        Vars
 		Tasks      Tasks
 	}
 	if err := unmarshal(&taskfile); err != nil {
@@ -33,6 +35,7 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	tf.Output = taskfile.Output
 	tf.Includes = taskfile.Includes
 	tf.Vars = taskfile.Vars
+	tf.Env = taskfile.Env
 	tf.Tasks = taskfile.Tasks
 	if tf.Expansions <= 0 {
 		tf.Expansions = 2