Use fs for language, add lang_files option

The local app translations are loaded, and then if [files]/lang_files
is provided (a directory containing custom translations), any found
inside it are loaded over top. This makes customizing much easier.

static.go

@@ -6,8 +6,10 @@ import (
 	"strings"
 )
 
+// Since the gin-static middleware uses a version of http.Filesystem with an extra Exists() func, we extend it here.
+
 type httpFS struct {
-	hfs http.FileSystem
+	hfs http.FileSystem // Created by converting fs.FS using http.FS()
 	fs  fs.FS
 }