Adds experimental flagging

[zmackie]

• Addresses Proposal: Feature flags #368

This global flag should support future development of runtime experimental feature, per the discussion in #368 .

pack report provides an example of how to consume that flag, in addition to reporting it's state:

± zm |feature/experimental → origin S:4 U:1 ✗| → ./out/pack report
Pack:
  Version:  0.0.0+git-27f8c87
  OS/Arch:  darwin/amd64

Default Lifecycle Version:  0.7.2

Config:
  default-builder-image = "heroku/buildpacks:18"
  experimental = true

  [[run-images]]
    image = "cnbs/test"
    mirrors = ["one", "two"]

  [[run-images]]
    image = "cnbs/test2"
    mirrors = ["one", "two"]