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/fs/tar.go

@@ -1,14 +1,14 @@
 package fs
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "tar",
 		Description: "Use archive file or device ARCHIVE",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "A", Description: "Append archive to the end of another archive"},
 			{Name: "c", Description: "Create a new archive"},
 			{Name: "b", Description: "Set record size to BLOCKSx512 bytes"},
@@ -41,7 +41,7 @@ func init() {
 			{Name: "bad-dumpdir", Description: "\\"},
 			{Name: "?", Description: "Display a short option summary and exit"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--force-local", Description: "Archive file is local even if it has a colon"},
 			{Name: "--rmt-command", Description: "Use  COMMAND instead of rmt when accessing remote archives"},
 			{Name: "--rsh-command", Description: "Use  COMMAND instead of rsh when accessing remote archives"},