Add external markup render support (#2570)

* add external markup render support

* bug fixed

* refacotr codes and fix wrong error log

* fix comments and add check to prevent leaks

* add check for config file and improve the example

* check file close error

* use ioutil.TempFile instead uuid

* correct Render -> Parser

* improve warning when incorrect markup setting

* fix typos

cmd/web.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/markup/external"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/routers"
 	"code.gitea.io/gitea/routers/routes"
@@ -59,6 +60,8 @@ func runWeb(ctx *cli.Context) error {
 
 	routers.GlobalInit()
 
+	external.RegisterParsers()
+
 	m := routes.NewMacaron()
 	routes.RegisterRoutes(m)