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/jvm/jenv.go

@@ -1,14 +1,14 @@
 package jvm
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "jenv",
 		Description: "Executable file",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "commands", Description: "List all available JEnv commands"},
 			{Name: "help", Description: "Parses and displays help contents from a command's source file"},
 			{Name: "info", Description: "Show information about which command will be executed"},
@@ -39,7 +39,7 @@ func init() {
 			{Name: "versions", Description: "Lists all Java versions found in `$JENV_ROOT/versions/*'"},
 			{Name: "whence", Description: "List all Java versions that contain the given executable"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--usage", Description: "Show information about which command will be executed"},
 			{Name: "--unset", Description: "Remove local jEnv settings"},
 			{Name: "--short", Description: "Show only files without path"},