Add optional fallback return value parameter

[tuexss]

As suggested in #149

[tenderlove]

LGTM. You mentioned in #149 that we should be returning nil by default. I think we should, but I'm unsure if I could change this return value in a bugfix release. I guess nil is still falsey, and the behavior isn't documented, so I'm not sure.

[tuexss]

Yes, I agree. Adding a feature is one thing, changing current behavior a different one. Maybe it should fail with a proper exception.

Do you have any idea what I can do about appveyor failing?

[sandstrom]

I just hit this. Getting nil in return is what I'd expect, and since both false and nil are falsy I wouldn't be to worried about changing in a bugfix release.

[sandstrom]

Nice! ⛵