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/which.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:        "which",
 		Description: "Executable file",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-s", Description: "No output, return 0 if all the executables are found, 1 if not"},
 			{Name: "-a", Description: "List all instances of executables found, instead of just the first"},
 		},