Add more control over log_format config directive

Introduces 3 new environmental variables:
 - `LOG_FORMAT`
 - `LOG_FORMAT_ESCAPE`
 - `LOG_JSON`

`LOG_FORMAT` and `LOG_FORMAT_ESCAPE` default to standard values.
When `LOG_JSON` is set, defaults are changed to: `{"time_local":"$time_iso8601","client_ip":"$http_x_forwarded_for","remote_addr":"$remote_addr","request":"$request","status":"$status","body_bytes_sent":"$body_bytes_sent","request_time":"$request_time","upstream_response_time":"$upstream_response_time","upstream_addr":"$upstream_addr","http_referrer":"$http_referer","http_user_agent":"$http_user_agent","request_id":"$request_id"}`and `json`.

See `nginx.tmpl` and https://nginx.org/en/docs/http/ngx_http_log_module.html#log_format for details

test/test_log_json_format.yml

@@ -0,0 +1,15 @@
+web1:
+  image: web
+  expose:
+    - "81"
+  environment:
+    WEB_PORTS: 81
+    VIRTUAL_HOST: nginx-proxy.test
+
+sut:
+  container_name: sut
+  image: nginxproxy/nginx-proxy:test
+  volumes:
+    - /var/run/docker.sock:/tmp/docker.sock:ro
+  environment:
+    LOG_FORMAT: '{"time_local":"$$time_iso8601","remote_addr":"$$remote_addr","request":"$$request","upstream_addr":"$$upstream_addr"}'