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/dpkg.go

@@ -1,14 +1,14 @@
 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:        "dpkg",
 		Description: "Debian package management system",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "query", Description: "Query the dpkg database"},
 			{Name: "install", Description: "Install or upgrade packages"},
 			{Name: "remove", Description: "Remove packages"},
@@ -20,7 +20,7 @@ func init() {
 			{Name: "check", Description: "Check the dependencies of packages"},
 			{Name: "compare-versions", Description: "Compare package versions"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-l", Description: "List packages matching a pattern"},
 			{Name: "-s", Description: "Display the status of available packages"},
 			{Name: "-L", Description: "List files in a package"},