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

[egargan]

Thanks @btnkr! Let's go 🥳

[egargan]

I don't have permissions to merge this by the way, so whoever does, please do!

[taoliujun]

can you merge it?