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/pg_dump.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:        "pg_dump",
 		Description: "Dumps a database as a text file or to other formats",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--help", Description: "Show help for pg_dump"},
 			{Name: "--file", Description: "Output file or directory name"},
 			{Name: "--format", Description: "Output file format"},