Add a "properties" section to apps/containers

[floitsch]

In the device specification: it would be really convenient to have a "properties" section with each application, that could then be used easily by the application.

Example (inside a device specification):

"containers": {
  "my_app": {
    "main": "foo.toit",
    "properties": {
      "pin1": 32,
      "pin2": 33
    }
  }
}

[floitsch]

There might be some restrictions we want to put on some of these properties (like rolling them back if the connection breaks). Should we always require a map for each property? Any other better way?