Add optional() helper

[andywer]

Currently, our object description looks something like this:

  dateOfBirth: parseDate(),
  addressLine1: string(),
  addressLine2: {
    validator: string(),
    optional: true
  }

I think it would be really nice to have fefe come with a convenience function, so that we write it like this instead:

  dateOfBirth: parseDate(),
  addressLine1: string(),
  addressLine2: optional(string())

Of course it would be trivial for the user to write such a function themself, but it feels so basic, that it would probably make more sense to ship it as part of fefe.

Cheers!

[andrenarchy]

This makes a lot of sense and I was already using this helper in other projects and I found myself copying it so this is the obvious solution. The same actually goes for a default() helper – however that is a keyword in js, any ideas?

[andrenarchy]

Added in 1.1.0. The default() helper is discussed in #7.