Basic gemspec

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Command line interface supporting module

Tainers::CLI provides command-line functionality for a to-be-provided
entry point script.

Test uses nasty intermingled shared example groups to make it easy
to test new commands with the standard options applicable to all
commands.

Basic expectation is that you provide a container specification as
you would to Tainers.specify, with --prefix and --suffix options
potentially overriding what you have in your specification.  You
provide the specification as JSON, via an arg (-j/--json), a
file (-f/--file), or stdin (the default).