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/sys/tsh.go

@@ -1,14 +1,14 @@
 package sys
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "tsh",
 		Description: "Remote host login",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "version", Description: "Print the version"},
 			{Name: "ssh", Description: "Run shell or execute a command on a remote SSH node"},
 			{Name: "user@hostname", Description: "Address of remote machine to log into"},
@@ -36,7 +36,7 @@ func init() {
 			{Name: "add", Description: "Add a new MFA device"},
 			{Name: "rm", Description: "Remove a MFA device"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-l", Description: "Remote host login"},
 			{Name: "--proxy", Description: "SSH proxy address"},
 			{Name: "--user", Description: "SSH proxy user"},