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/fs/stow.go

@@ -1,14 +1,14 @@
 package fs
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "stow",
 		Description: "Manage farms of symbolic links",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-n", Description: "Set the target directory to 'DIR' instead of the parent of the stow directory"},
 			{Name: "-v", Description: "Ignore files ending in this Perl regex"},
 			{Name: "--defer", Description: "Show Stow version, and exit"},