add UnmarshalPartially

[davidwtf]

In our project, we use hjson as part of configs, it's mixed with other format texts.
So we need read the part of hjson by hjson-go and keep the left part to other programs.
I added the function UnmarshalPartially to do this work.
UnmarshalPartially reads characters from the beginning, add try to parse them as hjson.
It stops when meeting wrong formats or finishing a complete hjson structure.
It returns the parsed value, the start position of the unprocessed part and error if has.

[dqsully]

This is a cool feature, but I'm not sure it fits in the official Hjson package. Maybe you could make a config format similar to how OctoberCMS does it?