v0.0.21: Major CLI improvements

Breaking changes in this version:

- All `<POD> <SERVICE>` arguments must now be written as either
  `<POD>/<SERVICE>`, or as just `<SERVICE>` if the name is unique.
- The `cage generate list` command is now called as `cage generate`.

New features include:

- A brand-new argument parser based on clap, with much better subcommand
  support.  Type just `cage` for detailed help.
- Experimental shell completion support for fish and bash.  Run
  `cage generate completion --help` for instructions.
- Support for `--entrypoint` and `-e` arguments.

To migrate an existing project, you will want to add a
`config/project.yml` file containing:

    # This is a semver requirement, and it supports the full syntax at
    # https://github.com/steveklabnik/semver
    cage_version: "0.0.21"

If your version of cage is too old or too new for a given project, it
will print an error.  This will make it easier for teams to keep up with
breaking changes in future releases of cage.

v0.0.20: Labels for override/pod & renewable vault tokens

We now add labels of the form `io.fdy.cage.override` and
`io.fdy.cage.pod` to all services, and our vault plugin generates
renewable tokens for better compatibility with `ecs-compose`'s smart
re-deploy features.

v0.0.19: Custom test commands

Instead of always running the full test suite using:

    cage test frontend web

You can now override the test command, and only run specific tests:

    cage test frontend web rake test:unit

v0.0.18: Finish renaming `cage2` → `cage`

We finish the renaming, including the GitHub repository and the binary
that we generate.  We also update the name of the release zip archives
to include the processor architecture.

v0.0.17: Attempt to fix binary builds

This contains minor fixes to the release code for v0.0.16.

v0.0.16: Begin the Great Renaming from "conductor" to "cage"

The crate is now known as `cage`, and the binary is temporarily known as
`cage2` until we finish a corresponding rename of the older version of
`cage` to `cage1`.

v0.0.15: Vault export, better errors and Mac build fixes

This release features substantial improvements to our Vault plugin,
better error reporting for some common errors, and some build fixes for
the Macintosh.

v0.0.14: Fix issues with `run` subcommand

We no longer pass `--` to `docker-compose run` or `exec`, to fix errors
with invocation of various tasks.

v0.0.13: Improve error reporting

This release includes a major error-handling overhaul to use
`error-chain` and to improve the quality of our error messages.  More
work remains to be done, but this should be a good start.

v0.0.12: Improve ergonomics in several small ways

Breaking changes:
- Rename `only_in_overrides` → `enable_in_overrides` in all config files
- Add `pod_type: placeholder` support for placeholder pods (db, etc.)
- Don't require `--override=test` for `test` command
- Don't start up any pods for `--override test up` unless they
  explicitly mention `test` in `only_in_overrides`