Support for Custom Mongo Query

How it looks:
-----

Normally:
![screenshot 2014-07-07 02 10 18](https://cloud.githubusercontent.com/assets/268126/3493299/8bc795d4-05b6-11e4-9f9f-c376a9fd5b8a.png)

Selecting custom query:
![screenshot 2014-07-07 02 14 07](https://cloud.githubusercontent.com/assets/268126/3493350/4717f478-05b7-11e4-9f9e-7314086da745.png)

Error in custom query:
![screenshot 2014-07-07 02 14 56](https://cloud.githubusercontent.com/assets/268126/3493353/4dec6a4a-05b7-11e4-8dc9-67c9046d65dc.png)


Yeah, that sounds good.