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```
This ought to be:
```
def context(self, mapping_class=None):
    mapping_class = mapping_class or DotDictWithAcquisition
    ...
```
Since `DotDictWithAcquisition` is based on `dict` as a base class, it's mutable and thus dangerous to have as a default argument.