Allow collection name mapping

[ratson]

Sometimes, collection name may not be in camelCase, it would be great if a mapping could be provided.
Allow object to be passed, e.g.

var db = mongojs('mydb', {
  collectionA: 'collection_a',
  collectionB: 'collection-b',
});

I original posted as then/then-mongo#5, find that it rely on mongojs implementation too.

[saintedlama]

I prefer to do something like this in my code

var db = mongojs('mydb', [
  'collection_a',
  'collection-b',
]);

module.exports = {
  collectionA: db.collection_a,
  collectionB: db.collection-b,
}

You can even do this if your on node 6+

var db = mongojs('mydb');
module.exports = Object.assign(db, { 
 collectionA: db.collection_a,
 collectionB: db.collection-b
}