Allow specifying an array of commands in the yaml

Hi. I tried upgrading from tmuxinator 0.5.0 to 0.6.5 today. Things were fine except one feature I was using. Version 0.5.0 allowed you to specify a list of commands for a pane. e.g.:
```yaml
windows:
  - shell:
    - git fetch
    - ls
 ```
These would then be flattened into the command `git fetch && ls`. This patch adds this functionality back in. It includes test coverage.

I placed the flatten_command method into Tmuxinator::Util. My reasoning was that it might make sense to allow passing an array of commands to `panes:` also, but I didn't add that in here. Let me know if anything needs to be moved around.