nix: add log_format option to the module

nix/module.nix

@@ -30,6 +30,7 @@ self: {
     config_dir = "${cfg.stateDir}/conf";
     cache_dir = cfg.cacheDir;
     disable_metrics = cfg.disableMetrics;
+    log_format = cfg.logFormat;
   });
 in {
   ###### interface
@@ -134,6 +135,12 @@ in {
         description = "Logging level for the Eris service.";
       };
 
+      logFormat = mkOption {
+        type = enum ["plain" "pretty" "json" "pretty-json"];
+        default = "pretty";
+        description = "Logging format for Eris.";
+      };
+
       user = mkOption {
         type = str;
         default = "eris";