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/view/xxd.go

@@ -1,14 +1,14 @@
 package view
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "xxd",
 		Description: "Make a hexdump or do the reverse",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-help", Description: "Show help for xxd"},
 			{Name: "-autoskip", Description: "Toggle autoskip: A single '*' replaces nul-lines.  Default off"},
 			{Name: "-bits", Description: "Switch to bits (binary digits) dump, rather than hexdump"},