commands: Make commands.Execute return a Response object

We have no global `Hugo` object no more (yay!), and there are some external tools that depends on that value.

These tools need to use get that value from `Response.Result`.

Note that `commands.Execute` now also takes the arguments as a string slice. This should also make it easier to use, not having to modify `os.Args`.

This commit also wraps up this particular issue. Phew!

Test coverage in /commands before: 14.4%
Now:  53.5%

Still work to do, now it is at least possible.

Closes #4598

main.go

@@ -19,22 +19,19 @@ import (
 	"os"
 
 	"github.com/gohugoio/hugo/commands"
-	jww "github.com/spf13/jwalterweatherman"
 )
 
 func main() {
 
 	runtime.GOMAXPROCS(runtime.NumCPU())
-	commands.Execute()
+	resp := commands.Execute(os.Args[1:])
 
-	if jww.LogCountForLevelsGreaterThanorEqualTo(jww.LevelError) > 0 {
+	if resp.Err != nil {
+		if resp.IsUserError() {
+			resp.Cmd.Println("")
+			resp.Cmd.Println(resp.Cmd.UsageString())
+		}
 		os.Exit(-1)
 	}
 
-	// TODO(bep) cli refactor
-	/*if commands.Hugo != nil {
-		if commands.Hugo.Log.LogCountForLevelsGreaterThanorEqualTo(jww.LevelError) > 0 {
-			os.Exit(-1)
-		}
-	}*/
 }