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/pkg_config.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:        "pkg-config",
 		Description: "Return metainformation about installed libraries",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--mod-version", Description: "Display the version information of the libraries specified on the command line"},
 			{Name: "--version", Description: "Display the version of pkg-config and terminates"},
 			{Name: "--atleast-pkgconfig-version", Description: "Require at least the given version of pkg-config"},