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/rust/rustup.go

@@ -1,14 +1,14 @@
 package rust
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "rustup",
 		Description: "The Rust toolchain installer",
-		Subcommands: []core.Subcommand{
+		Subcommands: []spec.Subcommand{
 			{Name: "show", Description: "Show the active and installed toolchains or profiles"},
 			{Name: "active-toolchain", Description: "Show the active toolchain"},
 			{Name: "home", Description: "Display the computed value of RUSTUP_HOME"},
@@ -37,7 +37,7 @@ func init() {
 			{Name: "auto-self-update", Description: "The rustup auto self update mode"},
 			{Name: "completions", Description: "Generate tab-completion scripts for your shell"},
 		},
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "-h", Description: "Prints help information"},
 			{Name: "--force", Description: "Force an update, even if some components are missing"},
 			{Name: "--force-non-host", Description: "Prints help information"},