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/sysctl.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:        "sysctl",
 		Description: "Variable name",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-A", Description: "List all the currently available non-opaque values"},
 			{Name: "-b", Description: "Force the value of the variable(s) to be output in raw, binary format"},
 			{Name: "-d", Description: "Print the description of the variable instead of its value"},