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/pkginstaller/pkgutil.go

@@ -1,21 +1,21 @@
 package pkginstaller
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "pkgutil",
 		Description: "Query and manipulate for macOS Installer packages and receipts",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "package-id", Description: "The package ID to list the files of"},
 			{Name: "path", Description: "The path to update ACLs on"},
 			{Name: "group-id", Description: "The group ID to list the packages of"},
 			{Name: "pkg-path", Description: "The path to the flat package to expand"},
 			{Name: "dir-path", Description: "The path to the directory to expand the package into"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--packages", Description: "List all installed package IDs on the specified --volume"},
 			{Name: "--pkgs-plist", Description: "List all of the files installed under the package-id"},
 			{Name: "--export-plist", Description: "The package ID to export the plist of"},