Add option to createIndex with background option

Hi,
Can you add support for creating Index in background?
https://docs.mongodb.com/v3.0/tutorial/build-indexes-in-the-background/

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/40734468-add-option-to-createindex-with-background-option?utm_campaign=plugin&utm_content=tracker%2F12293389&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F12293389&utm_medium=issues&utm_source=github).