Restore release (umbrella) publish support

[starbelly]

Obsoletes #240

This is the bare minimum to restore support for publishing from the top level of a release. That is to say, it essentially puts things back how they were vs including refactors that will happen in this module.

• Restore support for publishing multiple apps in a release
• Add --app switch which acts as the old --package switch did (required for reverting a specific app)
• Set the current_app in rebar state via rebar3_hex:task_state/1 so that rebar_state:current_app/1 may be utilized when not in a release.
• Set a multi_project k/v (boolean) in rebar3_hex:task_state/1

[starbelly]

I'd still refactor handle_task in publish module to be a single clause function and not rely on setting current_app -- just treat a single element list as the same as multi-element list of apps. But that isn't a reason to block this.

May end up doing just that, but let's see in my next PR if I can refactor such that the paths are clear, what's more if something goes wrong it's easily identifiable as belonging to a specific a path. It may be that I see after refactoring, this isn't worth it, it doesn't add any additional clarity.