add `is.keyed`

[mrDarcyMurphy]

Method would check that an object contains all the keys that the user is concerned with.

var obj = {
  foo: 'bar',
  fizz: 'buzz'
}
is.keyed(obj, ['foo', 'fizz']) // true
is.keyed(obj, ['foo', 'fizz', 'fart']) // false

[mrDarcyMurphy]

"foo" in obj would return true.

is.keyed(x, keys) {
  if (is.not.array(keys) || is.not.object(x)) return false
  return keys.every(function(key) {
    return key in obj
  })
}

[mrDarcyMurphy]

That'll do it, but I'll need to write more than a few tests to be sure. :sigh: