Add option to use returnDocument

[alecgibson]

Fixes #38

The mongodb Node.js driver deprecated use of returnOriginal in
favour of returnDocument in v3.6.

This non-breaking change allows consumers to opt in to using the newer
returnDocument by setting an option on construction

var queue = mongoDbQueue(db, 'queue', { returnDocument : true })

[sadortun]

Maybe renaming the function to getDefaultQueryOptions might improve readability

[alecgibson]

It's not quite what this function does: it actually adds the "return a new document" flag (whatever that is) to the passed set of options.

If we had access to the spread operator you could do something like:

var options = {
  // other options here
  ...self._returnNewDocument(),
}

...but we can't really do that without a transpiler I think.

[alecgibson]

Could just call it _returnOriginal() anyway. Probably just as meaningful with fewer words:

var options = self._returnNewDocument({
  // other options
})

[sadortun]

So I got a question, sorry for my ignorence, but why don't you just replace the flag and discard the previous/deprecated option?

If the usage is deprecated, just replace it ?

[alecgibson]

To avoid making this a breaking change — it would break any clients who are using older versions of mongodb.