Prefix option

[richardschneider]

Allow the key names to be prefixed with a custom string. The default is ''.

[timoxley]

@richardschneider wondering if perhaps this could be more generic as key and value mapping functions.

[richardschneider]

@timoxley I'm happy to make some changes. Could you provide a bit more details.

I added prefix so that an object could be flatten to a JSON pointer .

[richardschneider]

I've added the key mapping functions (keyname and keynames) for flatten and unflatten, respectively. Not too happy with the option names, perhaps you have better suggestion.