feat(client): add deis config:pull command

This command pulls in existing config variables set with `deis config:set` and writes them out to a local .env file for development purposes.

If a .env file exists and a environment variable in that file is present in the application's config, the file's envvar will be chosen unless the --overwrite option is selected. If --interactive is present, a prompt to write the environment variable to the file will be shown.

fixes #560

TESTING: run the supplied integration tests, verify that the `--interactive` displays a prompt, and verify that supplying `--overwrite` will overwrite a config variable in the local `.env` file.
We should be using docopt options syntax for this: https://github.com/docopt/docopt#option-descriptions-format