Make type casting configurable

I can see how the automatic casting of property values to JavaScript types might come in useful, but sometimes you really just want the literal value.

For my use case: I am reading a properties file containing mappings between file names and their SHA-1 hashes, truncated to 8 characters. Some of those hashes might look like numbers in exponential notation, say '90e02100', in which case having them cast to a Number is definitely not a good thing.

The same could be argued for numbers in octal or hex notation, I guess.

So, as per subject, I made this configurable, added the relevant tests and updated the Readme.