Feature request: Add two drawer support to `md-app` (left and right)

[crutchcorn]

I currently have an app that our designers have created a senario in which they feel two app drawers to be apt for, one of the left and one on the right. It would be phenominal to add support to md-app to support this functionality (as it currently throws errors informing developers that that functionality does not exist as of yet)

[RobertJGabriel]

Would love this support too !