This option receives an ARN of the role to assume and it makes the user
able to do the operations across the accounts according to the assume-role.

e.g.

```
$ ecspresso --help
Usage: ecspresso <command>

Flags:
  -h, --help                      Show context-sensitive help.
      --envfile=ENVFILE,...       environment files
      --debug                     enable debug log
      --ext-str=KEY=VALUE;...     external string values for Jsonnet
      --ext-code=KEY=VALUE;...    external code values for Jsonnet
      --config="ecspresso.yml"    config file
      --assume-role-arn=""        the ARN of the role to assume
      --option=OPTION

Commands:

...

$ ecspresso status --assume-role-arn=arn:aws:iam::123456789012:role/ecsOperatableRole
```

Signed-off-by: moznion <moznion@mail.moznion.net>

Add `--assume-role-arn` option to the top level of the CLI