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/where.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:        "where",
 		Description: "For each name, indicate how it should be interpreted",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-w", Description: "For each name, print 'name: word', where 'word' is the kind of command"},
 			{Name: "-p", Description: "Do a path search for the name, even if it's an alias/function/builtin"},
 			{Name: "-m", Description: "The arguments are taken as patterns (pattern characters must be quoted)"},