yes no prompt with a default

It is like agree, but follows our naming conventions
and it allows for a default

It does A, when I'd prefer B. But this is the way the prompting works in highline for prompts with a default value.
```
prompt? (Y/N): |Y| # A (this is what it does)
prompt? (Y/n):     # B (what I'd like)
```
Should we check `.blank?`  That is, if they type a space or tab and hit enter should it just pick the default?