add `oneOf` method

[mrDarcyMurphy]

Very much duplicating indexOf, still useful.

is.oneOf('subject', ['subject', 'standard']) // returns true
is.oneOf('fail', ['subject', 'standard']) // returns false

[mrDarcyMurphy]

Loose or strict comparison? Assume strict, for now. Initial use case is for the-thing-is where the configuration would be:

var thing = 'subject'
var whatIexpect = [
  'string', 
  { oneOf:['subject', 'standard'] }
]

if (the(thing).is(whatIexpect) {
  // ...
}

[mrDarcyMurphy]

Could also be aliased as contains.