Use Hoek to assign defaults to options passed to inject. Fixes #22

I wasn’t sure of the policy for dependencies so I used the same as what’s in hapi. Maybe I’m misunderstanding semver but it looks ok?

> On Dec 12, 2014, at 2:04 , Eran Hammer <notifications@github.com> wrote:
> 
> In package.json <https://github.com/hapijs/shot/pull/23#discussion-diff-21730785>:
> 
> > @@ -13,7 +13,9 @@
> >    "engines": {
> >      "node": ">=0.10.30"
> >    },
> > -  "dependencies": {},
> > +  "dependencies": {
> > +    "hoek": "^2.10.0"
> why ^?
> 
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