Feature request: only one function for loading files

Hi,

another idea which came into my mind is to provide just one function for loading data into an instance. Currently there are a number of class methods for creating a new instance with a type of file, i.e. `from_json`, `from_yaml`, etc.

This could be simplified with one function and parse the file according the extension or an optional keyword parameter:
```python
>>> config = Box.from_file('/this/is/my/datafile.yml')
# or
>>> config = Box.from_file('/this/is/my/datafile.txt', file_type='yaml')
```
Just an idea, I had a similar implementation in my own config parser, but not as sophisticated like your package `python-box`. It's really, really great, thanks for the efforts.

Regards, Thomas