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/ops/fnm.go

@@ -1,14 +1,14 @@
 package ops
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "fnm",
 		Description: "Fast and simple Node.js version manager",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "install", Description: "Install a new Node.js version"},
 			{Name: "uninstall", Description: "Uninstall a Node.js version"},
 			{Name: "use", Description: "Change Node.js version"},
@@ -24,7 +24,7 @@ func init() {
 			{Name: "env", Description: "Print and set up required environment variables for fnm"},
 			{Name: "help", Description: "Prints the help page or the help of the given subcommand(s)"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--lts", Description: "Install latest LTS"},
 			{Name: "--install-if-missing", Description: "Install the version if it isn't installed yet"},
 			{Name: "--help", Description: "Prints help information"},