Cleanly exits from commands.

This captures interrupts(ctrl + c) when running the `start` or `setup` commands and  lets them close cleanly without printing the exception to the terminal.

I get this when using ctrl + c.

```bash
^C/Users/kyuden/pj/pr/fluentd-ui/lib/fluentd-ui/command.rb:11:in `system': Interrupt
        from /Users/kyuden/pj/pr/fluentd-ui/lib/fluentd-ui/command.rb:11:in `start'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
        from /Users/kyuden/pj/pr/fluentd-ui/bin/fluentd-ui:13:in `<top (required)>'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bin/fluentd-ui:23:in `load'
        from /Users/kyuden/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bin/fluentd-ui:23:in `<main>'
```