Create and deploy PowerApps from JSON definition

[LarsFastrup]

PowerApps is great for power users. But as a developer I would also like to see these features:

• Export existing PowerApps to a JSON definition file much like an ARM template in Azure.
• Create and update PowerApps from a JSON definition file.
• REST API and PowerShell cmdlets for deploying and managing PowerApps in the organization.

It would make a much better developer story for testing, staging, deploying and versioning PowerApps.

[Audrie-MSFT]

@CarlosFigueira as FYI