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/git/git_cliff.go

@@ -1,14 +1,14 @@
 package git
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "git-cliff",
 		Description: "Increases the logging verbosity",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--verbose", Description: "Increases the logging verbosity"},
 			{Name: "--init", Description: "Writes the default configuration file to cliff.toml"},
 			{Name: "--latest", Description: "Processes the commits starting from the latest tag"},