Maintain type when overriding values via env var and argv

[youming-lin]

Env var and argv will assign non-JSON, non-PLIST to String type, always. This makes overriding numeric values with numeric values impossible. We should figure out a way to coerce the string values from env var and argv into the correct types.

[Fryie]

I have also run into this issue, it's also a problem for e.g. boolean values.

[Fryie]

A "workaround" is to instead supply a JSON dictionary for the parent key with the property to override as a single key. I believe, the library will then merge this dictionary with the dictionary for the parent key derived from other sources.