extended mapping_class for config to function & context

the method ```ConfigurationManager.get_config``` accepts an object hook so that the users can specify what type of mapping container in which they want their configuration.  That method isn't the only way to get configuration out of configman.  

This PR extends the mapping object_hook to the two other ways to get configuration:  ```ConfigurationManager.context``` and the bare function ```configuration```
I would prefer something like:
```python
if 'mapping_class' in kwargs:
   mapping_class = kwargs.pop('mapping_class')
else:
    mapping_class = None
...
if mapping_class is not None:
    return cm.get_config(mapping_class=mapping_class)
else:
    return cm.get_config()
```

Reason being that something could go wrong *inside* `get_config()`. Like a `NameError` and then you don't want to catch the wrong exception for the wrong reason. I personally prefer to only use try:except: when there is only one thing that can go wrong.