Add a command parameter and support arrays for command and shell

[lox]

It dawned on @toolmantim and I that a good solution to the question of whether this plugin and docker-compose should default to a shell is the following:

• If a command is provided via the step-level command, assume a shell, as we tend to do things like yarn install && yarn start in examples:

steps:
  - command: yarn install && yarn run test
    plugins:
      docker#v1.4.0:
        image: "node:7"
        workdir: /app

The above will translate to /bin/sh -e -c 'yarn install && yarn run test' in the container. This will also support multiple line commands.

• If no step level command is provided, no shell is used. If you want control over exactly what command is passed to docker (e.g how the command is split into arguments), you can use the new plugin level command this PR introduces:

steps:
  - plugins:
      docker#v1.4.0:
        image: "koalaman/shellcheck"
        command: ["--exclude=SC2207", "/app/script.sh"]
        workdir: /app

This avoids us having to do any shell word parsing, which is horrible for complexity and security.

The new array syntax is also applied to entrypoint and shell:

steps:
  - plugins:
       docker#v1.4.0:
         image: "alpine"
         command: ["echo", "hello", "world"]
         shell: ["/bin/sh", "-c"]

[lox]

This is ready to go if you get a chance to review it @toolmantim or @matthewd

[toolmantim]

🎉