Enable directory listing in JSON

[Nowaker]

Dirlisting is a good thing but should be disabled by default.

JSON format should match nginx's:

[
{ "name":"virtsimple-reseller-whmcs-addon-v4.zip", "type":"file", "mtime":"Wed, 20 Apr 2016 19:33:29 GMT", "size":158319 },
{ "name":"virtsimple-reseller-whmcs-addon-v6.sha256", "type":"file", "mtime":"Wed, 20 Apr 2016 19:48:28 GMT", "size":108 },
{ "name":"virtsimple-reseller-whmcs-addon-v6.zip", "type":"file", "mtime":"Wed, 20 Apr 2016 19:48:28 GMT", "size":158319 }
]