Add support for Mongo 2dsphere indexes

From this example:
https://docs.mongodb.com/manual/tutorial/geospatial-tutorial/#prerequisites

It would be nice if db-migrate could support this:
`db.restaurants.createIndex({ location: "2dsphere" })`

This currently can't be done with `addIndex` NoSQL API.

Workaround: loading native mongodb driver:
```js
const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');
const url = 'mongodb://mongo:27017/test';

exports.up = function(db, callback) {
MongoClient.connect(url, function(err, dbnative) {
assert.equal(null, err);
assert.notEqual(null, dbnative);
dbnative.collection('ubicaciones').createIndex({ location: "2dsphere" })
dbnative.close();
callback();
});
return null;
}

---
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