Add chainable indempotent 'not' method to StringInquirer.

This is just a little sugar on StringInquirer, but
- StringInquirer is already just sugar
- I've been using this patch often enough, long enough I figured others might like it

I use StringInquirer extensively, most frequently in non-Rails apps to wrap ENV variables into meaningful predicate methods on various classes and modules in my library. This often leads to code like

`if not Git::Repository.host.github?`

Since this is sugar anyhow, I prefer the more natural

`if Git::Repository.host.not.github?`

Discuss/dismiss away.