add yes as an option to userinput
add yes as an option to userinput

Summary:
I found it kind of confusing when yes was mentioned in a dialog but didn't work.

So maybe add it to the userinput?

maybe that works in this repo?

Test Plan:
```
$ ./bin/buck doctor
<snip>
Is buck not picking up changes to files? (saying 'yes' will run extra consistency checks) (Y/n)?
yes
(most output is going to your pipe, some short summary will show here on stderr)

$ ./bin/buck doctor
<snip>
Is buck not picking up changes to files? (saying 'yes' will run extra consistency checks) (Y/n)?
no
Writing report, please wait..
```

Reviewed By: philipjameson

fbshipit-source-id: abe12ea

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