Add Json mode to mod_autoindex

[andrii-suse]

The code change is in 0a97bbc The rest commits are to demonstrate that the added code works properly: 0d4363e and 766396e are from #165 , where environs tests are proposed 6d0229a adds environs test for 0a97bbc : https://github.com/andrii-suse/httpd/runs/1662328296?check_suite_focus=true#step:7:682

2c53bf3 is needed to make travis tests pass due to fix in #165; to be removed before merge.

Feel free to accept only 0a97bbc if the integration tests raise too many questions.