Added the ability to reuse a MongoClient instance

[boycce]

Use case

This is helpful when you want to use Monk within migrate-mongo migration files where you are supplied with a MongoClient instance in their methods.

Example

monk('127.0.0.1/my-db', (err, db) => {
  monk(db._client, (err, db2) => {..})
})

[boycce]

Could this potentially be merged if i fix the errors?

[mathieudutour]

sure