Command type returns stdout, so you can use any matchers you like to the stdout

This is an sample implementation to meet the demand of @rickhull in #276.

You can write spec like this.

```ruby
describe command('ls -al /') do
  its(:stdout) { should match /bin/ }
end 
```

You can use any matchers that rspec has.And the command runs only once because its stdout is cached.