Add `--out file_path` to specify multiple formatters and outputs, including tests.

I set this up very similarly to the way rspec, cucumber, and rubocop work.

The default reporter remains the same, and the default output is `:stdout`, which is lazily mapped to `$stdout` at the execution of the lints. Specifying an `--out file_path` flag will map the previously specified reporter results (or default reporter if no `--format` flag specified) to the file_path. So, for instance, specifying an additional `--out file_path` flag without a preceding `--format Reporter` flag will overwrite the previous default reporter path.

Also, one final note - there may be a more appropriate place for this default value to go:

```
# lib/scss_lint/cli.rb:24
    def initialize(args = [])
      @args    = args
      @options = { reporters: [[SCSSLint::Reporter::DefaultReporter, :stdout]] }
      @config  = Config.default
    end
```

I included tests in the cli_spec.rb. I'm hoping this is sufficient. Assuming we can get this merged, I have a [`guard` runner gem](https://github.com/arkbot/guard-scss-lint) ready to make use of the newly available CLI flags.

Let me know if you need anything else. Cheers.