Generate default shared schemes?

At the moment shared schemes are only generated if they are defined in target.scheme or schemes. The problem is if someone wants to generate a simple shared scheme with no special options, the best way is to specify a target scheme with no options which looks a bit weird.

targets:
  MyTarget:
    scheme: {}

This issue is highlighted here https://github.com/yonaskolb/XcodeGen/issues/57

When Xcode opens a project it creates default private schemes for each target if they don't exist already. What we could do is generate a default shared scheme for each target, possibly just app targets. You could still provide options for this via target.scheme, and if a top level scheme with the same name was created, it would override this default scheme.
Possibly this could even live behind an option. Thoughts?

No source file or test is named. Start by locating the code that handles target.scheme and top-level schemes, then trace how shared schemes are emitted. Define the scope for default generation and any option needed; done means simple shared schemes no longer require an empty scheme declaration while explicit schemes still override defaults.