Change to merge data from config file into config object

I load both a `common.json` config file and an environment-specific config file `development.json`. The config contains values that are hashes, and when I load the `development.json` config file, these hashes are overwritten rather than merged. 

This PR merges the data from the config file into the config object so that hashes aren't overwritten.

I've used `object-extend` module, but I can copy the merge function in if you don't want the dependancy.

I initially tried to do this outside the module, but it's a bit clumsy because I had to `deepClone()` the common config in order to merge it with the environment config.