Add hidden_params for logging

[surik]

This PR adds two options for logging:

1. logger_formatter: json | native
   How to print request and response in logs. json by default

2. hidden_params: [atom() | binary() | [atom() | binary()]]
   Which keys will be hidden. For example we have request like this:

{
 "key1": "value", 
 "key2":  {
     "key3": "value"
 },
 "key4":  {
     "key5": {
         "key6": "value"
     }
 }
}

and we want to hide key1 and key6, we should use hidden_params = [key1, [key4, key5, key6]].
[] by default

Relate to #58