ask: add support for default answer.
Default answer defaults to nil.

As @trans pointed out: not not all questions are necessarily "Yn".
And if default answer is nil, one can do

```ruby
if answer = ask("Some question?")
  # do something with answer
else
  # no answer
end
```
Merge pull request #2 from weakish/ask-default-answer

Add support for default answer to ask.