add option to translate all ObjectID fields per record

[crobinson42]

#16

• - tests

[jmdobry]

I personally do not use MongoDB, so can you explain what this PR does? I just want to make sure there aren't any breaking changes.

Also, these changes need tests. (Tests would help explain the expected behavior.)

[crobinson42]

I ran into the need for this feature when starting to implement js-data-mongodb for an API that uses an existing MongoDB database set of data. The other API's in production for this database would set fields that have relation to another record not by simply the string id but by new ObjectID(idString) - when fetching records with js-data using js-data-mongodb I was expecting a record like this:

{
 id: "57bc950fe77ea192aece82d6",
 name: "Cory"
 // a relation field storing the id of the company record
 company:  "57bc9541e77ea192aece82d7"
}

But, instead I was getting this:

{
 id: "57bc950fe77ea192aece82d6",
 name: "Cory"
 // a relation field storing the id of the company record
 company:  {
  _bsontype: "ObjectID", 
  id: "<Buffer 35 37 62 63 39 35 34 31 65 37 37 65 61 31 39 32 61 65 63 65 38 32 64 37>"
 }
}

This PR gives the option to convert any ObjectID bson fields in a MongoDB record to a string id.

You can experiment with the above example in your node console:

const ObjectID = require('bson').ObjectID
console.dir(new ObjectID())

You'll most likely see funny characters on the id field of that object, that's because it's a Buffer object. Anyways, that demonstrates the use and need for this PR.