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/yank.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:        "yank",
 		Description: "Yank terminal output to clipboard",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-i", Description: "Ignore case differences between pattern and the input"},
 			{Name: "-l", Description: "Use the default delimiters except for space"},
 			{Name: "-x", Description: "Use alternate screen"},