Support for umbrella projects

[arturz]

In order to support umbrella projects there has to be an option to group contexts. Cross-references would only be checked within each context subgroup.

Syntax proposal:

config :contexted,
  contexts: [
    [First.FirstContext, First.SecondContext],
    [Second.FirstContext, Second.SecondContext]
  ]

[rmatteoda]

Maybe you can configure :contexted in the config file of each app inside umbrella apps instead of using the global config for all contexts? Not sure if using that option the feature that checks cross-references worked as expected or still needs some change.

Another possibility is to add an option in_umbrella: true in the config of :contexted to be detected by the library and run Cross-references for that use case with context grouped by app.

I am open to discussing this feature and contribute with a PR.
Best