Add function maps:get/3 for return default value

Convenient maps to use in configuration files, but specify default values ​​code is not convenient, we have to do wrappers.This function allows you to specify a default value.

```erlang
5> Map = #{ key1 => val1, key2 => val2 }.
#{key1 => val1,key2 => val2}
6> maps:get(key1, Map, "Default value").
val1
7> maps:get(key2, Map, "Default value").
val2
8> maps:get(key3, Map, "Default value").
"Default value"
```