refactor(commands): separate commands and its core into two folders (#32)

- I think it would be better if one for commands only, one for the spec
handling core logic so people won't be confused by the core logic inside
the commands folder (cleaner I guess)
- Regen docs

commands/ops/terragrunt.go

@@ -1,20 +1,20 @@
 package ops
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "terragrunt",
 		Description: "Workspace",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "show", Description: "Display the current workspace"},
 			{Name: "list", Description: "List the workspace"},
 			{Name: "delete", Description: "Delete the specified workspace"},
 			{Name: "select", Description: "Change the current working workspace"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-lock", Description: "Lock the state file when locking is supported. Defaults to true"},
 			{Name: "-force", Description: "Delete the workspace even if its state is not empty. Defaults to false"},
 			{Name: "-lock-timeout", Description: "Duration to retry a state lock. Default 0s"},