add 'objectProto' option to create parsed objects as {}

[egargan]

Fixes #39.

This gives us the option of going back to the old object parsing behaviour, before c85a430, where any parsed object was created as {}.

[guilleasz-glovo]

This PR seems really useful, is there any plan to merge it?

[Targma]

I really need this functionality, else we need to wrap function with Object.assign({}, parsedObject). Performance is important in project and with this we would avoid another copy.

[egargan]

I've emailed the owner but haven't heard anything back, we might be waiting a while!

[btnkr]

still hope merge it