Support app_service and application_service_plan

[Nils Woxholt]

Any reason why these are not included here?

I see them mentioned in the docs/defined_specs as:
app_service_plan = "plan"
web_app = "app"

Although a web_app refers to TF resource "azurerm_app_service" so should be called app_service?

If they are missing, I'll go ahead and add them :)

[Nils Woxholt]

Submitted PR #46 to solve this