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/tfenv.go

@@ -1,14 +1,14 @@
 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:        "tfenv",
 		Description: "Version",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "install", Description: "Install a specific version of Terraform"},
 			{Name: "version", Description: "Possible Terraform Version"},
 			{Name: "use", Description: "Switch to a version to use"},
@@ -18,7 +18,7 @@ func init() {
 			{Name: "version-name", Description: "Print current version"},
 			{Name: "init", Description: "Update environment to use tfenv correctly"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-v", Description: "View your current tfenv version"},
 			{Name: "--help", Description: "View commands"},
 		},