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/tac.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:        "tac",
 		Description: "Concatenate and print files in reverse",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--help", Description: "Display this help and exit"},
 			{Name: "--before", Description: "Attach the separator before instead of after"},
 			{Name: "--regex", Description: "Interpret the separator as a regular expression"},