Extend environment support for "soft aliases"

[joelgallant]

trafficstars

Meaning specific environments that are very similar to another, with only a few values different

• Explore option to $extends w/ a different environment than current one
• Explore "combined" environment (ie. test.dev), which merges two environments, allowing for holes in them individually

[joelgallant]

Going to say this should be done with #131

[joelgallant]

I think there's still value in a qa+staging or qa|staging syntax in $env. Reopening.

Partially, I think we should reserve this syntax by adding a deprecation warning for environment names with special characters (+, |, & should suffice).

[joelgallant]

Note that it's possible to do this (now) with at the root level of the app-config file:

$env:
  default: {}
  qa:
    $extendsSelf:
      env: staging
      select: .

    # any qa overrides go here