Change `env-file` parameter type from string to array

As @michaelsauter suggested (https://github.com/michaelsauter/crane/issues/94#issuecomment-54230522) I made the `env-file` parameter to be handled the same way `env` param is.

Usage example:
```yaml
---
containers:
  testing:
    image: ubuntu
    run:
      cmd: bash
      env-file:
        - tests/1.env
        - tests/2.env
      interactive: true
      rm: true
      tty: true
```