Feature request: add additional values from file

Hi,
I'm really happy that I found your excellent package, it makes my life a lot easier. One thing I may miss is the possibility to load extra attributes from a file into an existing instance.

Example:
I load a main configuration file and want to add additional configuration values from a plugin configuration file into the same Box instance. I created a much smaller solution myself, where I added a function `add_from_file` and instead of creating of a new instance I simply updated the existing one with the values coming from the second file.

This may be easier than creating two independent instances and then updating the first one with the values from the second one.
What do you think?

Regards, Thomas