allow "pretty" format for logging into files

ref #88

lib/logger.js

@@ -42,8 +42,12 @@ module.exports.setup = function(logs) {
 				Logger.emit('error', err)
 			})
 			stream.write = function(obj) {
+				if (target.format === 'pretty') {
+					dest.write(print(obj.level, obj.msg, obj, false) + '\n')
+				} else {
 					dest.write(JSON.stringify(obj, Logger.safeCycles()) + '\n')
 				}
+			}
 		} else {
 			throw new Error('wrong target type for a log')
 		}